No problems about not having a thermometer although it might be helpful at some point to know if he's running a fever.
Gums should be pale pink which I assume is their color which is good. Moist is also good since I would worry about dehydration.
It's possible that there are several different explanations for his behavior:
1. Constipation could certainly be responsible for many of his symptoms although I don't have a good explanation for the bruising. This wouldn't be expected for a cat with this problem. I have several suggestions if you think this may be what's causing his issues now. (see below)
2. To be honest, I always become a little concerned when a cat this age starts to vomit, becomes lethargic and now has disinterest in a food. I worry that there's some underlying systemic disease such as kidney issues or problems with the liver or pancreas which he's been able to hide and is now catching up with him.
Blood work in most cases would help identify these problems.
3. It would be a little unusual for a cat this age to have a foreign body causing an obstruction but the symptoms would be similar.
4. Fevers can cause lethargy and disinterest in food but not usually vomiting. If you manage to get your hands on a thermometer, you can try to take his temperature although this isn't always the easiest thing to do in a cat. Normal is between 100 and 102.5.
5. I'm not certain how the bruising or discoloration connects to his other symptoms. If we take one worst case scenario, for example and assume he was hit by a car, I would expect him to be much sicker, especially he has internal injuries.
A low platelet count can cause bruising but it would be quite unusual for this to happen in a cat. It's quite a rare condition but it could cause what looks like bruising under the skin. More often than not, this condition is secondary to viruses such as Leukemia, Feline AIDS, Feline Infectious Peritonitis; immune related conditions are extremely rare in the cat.
It sounds like he may be getting worse, but I do have a few suggestions which you can try at home to see if you can get him to feel better:
1. You can give him Pepcid AC at a dose of 1/4th of a 10 mg tablet twice a day.
a) If this drug is hard to find, then Zantac (Ranitidine) can be given although it's a very small dose...1/8th of a 75 mg tablet twice a day.
b) Another option would be Prilosec (Omeprazole), 1/4th of a 10 mg tablet once daily.
c) or Dramamine can sometime be effective as an anti-vomiting agent. Dose would be 12.5 mg twice a day or 1/4th of a 50 mg tablet.
d) Meclizine (Antivert) 12.5 mg once daily
If you can't pill him, then mix the drug with a small amount of water and use a syringe or eye dropper to squeeze it into the side of his mouth where there's a gap.
2. If you wanted to give him laxatives on the off chance that he's constipated, you also have options:
a) Hairball laxatives such as Laxatone or Laxairewhich are brand names but every pet or grain store will carry these products. To be honest, they may not be strong enough for this problem now.
b), Canned pumpkin (NOT the pie mix) at a dose of 1/4-1/2 tsp per day.
c) . Over the counter, human Miralax at a dose of 1/8th-1/4th tsp daily
d) Pediatric suppositories can be given (LINK) but this may be more than he will tolerate.
3. If he stops vomiting, then you can offer boiled chicken or human baby food but avoid those with onion and garlic. I'd also offer him tuna or clam juice or even low sodium chicken broth. It's going to be important to push fluids if he's running a fever. Pedialyte can also be offered but many cats don't like the taste.
I'm a little concerned that the longer he continues with these symptoms, the worse he might become, so if he's not showing improvement within another day, then I'd consider having him seen.
I realize that my answer may not be what you want to hear but I would be doing a disservice to both you and Enzo if I were less than truthful and honest in my response to you. I hope you understand.
I also hope that this is helpful and that Enzo will feel better soon. Deb