Assuming he is an entire male (has not been castrated), sadly this is a relatively common occurence. These cats get into all sorts of mischief because of their testosterone.
They will mate with females and fight with makes, commonly catching infectious disease such as feline Aids and Feline Leukaemia.
They are also prone to injuries such as being hit by cars and falling from heights, as well as cat fights.
On top of this, he is likely unvaccinated so prone to cat flu.
I would suspect he may have been injured or has one of the diseases mentioned above. Of course, there are other potential causes for his signs but these are most likely.
Ideally he needs to be seen by a vet ASAP so we can figure out what is going on and can start therapy.
The main concern is his breathing which may indicate e.g. fluid in his chest. Struggling to breathe can be quickly fatal so we want to get him sorted.
As he is difficult to get into a carrier to bring to the vet consider the following:
-Have a vet come to your home
-Ask a charity such as the RSPCA to send someone to get him
-Ask a charity to trap him in a humane cat trap and then have him assessed by the vet
-Work on getting him in the carrier by e.g. tempting him with food, so you can bring him to your vet
Depending on what is found, he may be rehomeable (to you or someone else), but we need to get him healthy first.