Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
First, I would note that the gas and his loose stools are likely being caused by the same issue. So, settling one will address the other. Furthermore, while it is possible that this lower GI upset was originally precipitated by Gene's diet change (if this was done quickly and he was not weaned over to the new diets slowly); the length of time this has been an issue does raise concerns of something more significant being present in his GI.
Now as I am sure you can appreciate, GI upset of this nature in the recently adopted cat is not uncommon. The reason is because these wee ones are undergoing a significant amount of life change (new house, new owners, new smells, new furniture, recent neutering, etc) which is a bit stressful. And when the body is under stress of any kind it releases stress hormones which have a side effect of dampening the immune system (the same reason students tend to get ill after final exams). This means if the kitty was fighting off a GI bug, stress can give the agent the break it needs to gain a foothold and cause clinical signs like diarrhea.
With this in mind, as I am sure you can appreciate, just like people, cats can have diarrhea that is caused by a range of agents. These include bacterial viral, parasitic (worms but also protozoa like Giardia, Coccidia, Tritrichomonas), toxin exposure (less likely here), nutritional issues and general dietary indiscretions causes. It can be daunting to get to the bottom of which differential is to blame, but since it sounds like he is otherwise normal and the diarrhea is not profuse, there are a few things you can try at home try and get it to settle.
To start, the first thing you can do is offer a light/easily digestible diet. This will be easier on his compromised gut then the current hard food diet he is on. Examples of this would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used in cases of gastroenteritis (ie Hill’s I/D or Royal Canin’s sensitivity). These diets aim for provide nutrition to the stressed gut without making it work hard to digest the food. I would advise frequent small meals to minimize strain on the already stressed gut, and to help lower the volume of diarrhea. Once he is settled, you can try weaning him slowly back onto the diet of your choice (and if it relapses, then an alternative diet may be necessary).
Furthermore, while I would hope he has been recently wormed by the rescue, if they did not then you'd want to consider doing so now. Ideally, you want to use a good quality wormer that will cover against all GI worms. Examples would be Panacur, Drontal, or Milbemax as it will cover all the worms in question. These are available over the counter at the vets, pet stores, and some pharmacies. Do make sure you have an idea of Gene's weight before purchasing, so you make sure to get the right dose for his size
On top of all this, we do also keep a close eye on his water intake and his hydration. To check his hydration status to make sure he is not becoming dehydrated there are a few things we can test at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether the pet has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. ( They use a big dog but it makes it easier to see and the principles are exactly the same) If you are seeing any signs of dehydration already, then you do want to have your kitty seen by his vet before this gets out of control for him.
If you are concerned that he is becoming dehydrated from the fluid loss via diarrhea, you can try and encourage him to drink but offering fresh water or even low-sodium chicken broth. If he isn’t amenable to these, you can syringe feed pedialyte. Pedialyte is good here (though aim for a flavourless one since cats don’t love fruit) because it will get both fluids and lost electrolytes back into your kitty. A typical maintenance rate for hydration in an animal is 48mls per kilogram of weight a day. If you do give syringe pedialyte, this should obviously be divided up into multiple offerings through the day rather then all at once. This value will give you the total he needs for the day and is a good starting point to give you an idea of the feline daily requirement. (we aren’t calculating losses, so you can add an equivalent volume to match how much diarrhea is being producing). If he vomits when you have given pedialyte, then therapy should be discontinued (since we don’t want vomiting because of our intervention).
Finally. if a cat has diarrhea just due to benign GI upset, the above is usually enough to settle it for them. But sometimes we need to use feline safe anti-diarrheals to slow things down for their gut. As I am sure you appreciate, these would not be a cure (since cures would depend on the culprit and might include antibiotics or anti-parasitics, etc.) but would slow the diarrhea to aid the body potentially absorb more water/nutrients then it would have if the diarrhea were unchecked. In regards ***** ***** options for him, the one we most commonly use in cats is Kaolin (More Info/Dose) available from your local pharmacy or Protexin Pro-Fiber (which is available OTC at vet practices; example). Both will slow diarrhea safely for a cat and the Pro-Fiber has the bonus of providing support to the delicate good bacteria of the GI. And just to note do avoid using any Pepto Bismol (or similar products with aspirin in them) or Loperamide/Immodium (as this can cause adverse signs in cats).
Overall, diarrhea can be caused by a wide range of agents. If he is otherwise well in himself, properly hydrated, and eating/drinking; then you can try the above supportive care to see if you can settle his signs. If you try the above and the diarrhea and gas are not settling in 24- 48 hours, then do consider following up with his vet, ideally with a fecal sample. They will be able to assess the diarrhea and send it for testing if need be. They will also be able to examine him to make sure this diarrhea isn't due to something sinister, address any dehydration issues, and cover him with broad spectrum antibiotics to clear any common GI bacterial gastroenteritis that may be causing this diarrhea and get him back on track as soon as possible.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,