Thank you Gill,
First, worms are a common cause of increased appetite and weight loss in the middle aged cat. And while worming a stray monthly (since he will likely be supplementing his diet with wildlife) is ideal, if he was wormed within the last 3 months and is still desperately hungry then worms would be less likely our main culprit here.
Instead, we have to consider other health issues (ie diabetes, hyperthyroidism, organ based issues, polyphagia secondary to nutrition loss should he also have diarrhea, etc) and behavior causes. And I would note that as long as he does appear fit and not losing weight, I'd be less inclined to be worried about health issues at this stage. Furthermore,
I would also note that many stray or former stray cats will display a greedy type appetite without clinical disease. This is because life as a stray doesn't come with guaranteed meals like pet life does. Therefore, the stray cat often will shovel all they can in when they can, until they finally realize that food is no longer scarce (and some rehomed strays never quite get it and therefore cannot have free feeding in their new homes). So, we do have to conisder that while there may be a pathological reason for his ravenous appetite, if he is keeping weight on and is in good condition this could also be behavioral.
That all said, the rapid breathing is going to be the bigger issue here and is not likely related to his appetite situation. Now if his respiratory rate is >30 breaths per minute (you can could his breaths for 10 seconds + multiply by 6 to obtain his rate), then we'd have to consider that he may have a lung based issue. The most common to affect our strays is going to be the cat flu agents (which are a risk for your cats too). And it is quite possible to only see lung signs in adult cats with this (where kittens often have all the signs due to having naive immune systems). Less common reasons we'd have to also consider for William would be pneumonia, chest trauma (if he has been hit causing contusions in the lungs or damage to his diaphragm), viral infections, heart issues, lung worm, blood clotting issues (if he has been exposed to rat bait), and asthma type issues.
Finally, I would just note that fighting through the cat flap isn't unexpected here. William will be getting used to being fed on your land and will naturally want to extend your area as part of his territory. The only problem is that cats are a territorial species and if they move into an area, they will often be inclined to drive any "competitors for food" away (your cats). And this means he won't be overly keen on your other kitties being there. Some cats can co-exist peacefully but generally speaking females aren't keen to share territory nor are entire male strays (who are living as close to natural cat living as they can in an urban area). Therefore, the behavior between the 2 is not unexpected. We may find they settle in time but at the moment with his respiratory issues, I have to say that you do not want him in any contact (even sharing an air space) with your cats.
Now in this case, what to do all depends on that respiratory rate. If it is above normal, then we'd need to think about catching him to get him help. I appreciate you have tried but in this situation, I would say that it is best to get your local cat charity involved (ie
Cat's Protection -- http://www.cats.org.uk/). They will have humane cat traps to properly catch him, vets on staff to examine him, and the facilities to treat him (since you will likely be hard pressed to medicate or provide supportive care for him). And while the appetite is what raised your concern, its this breathing that is our biggest concern and needs to be addressed for William.
I hope this information is helpful.
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All the best,
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