So glad to find this thread. The stray I've rescued recently is super tiny. I had thought she was 6-7 months old but after I caught her, the vet told me she was at least 1-2 years old and weighing a little under 5 pounds!! I found that hard to believe, but after reading this forum, I guess it's not unusual. I don't think it is a particular breed, just the result of malnutrition, early pregnancy, or simply being the runt of a litter. It is great to read that most live a full life with little issue, and it is also pretty cool that they remain tiny... not that I prefer tiny, but after having 3 enormous cats (not fat, but just huge bone structures weighing 15-17 pounds average - like little panthers), having a teenie cat is a neat change.
I think you shouldn't worry so much about the size as much as if he's protected from animals, elements, and starvation. Maybe the only thing to consider is trying to trap him and get him fixed and vaccinated, if you haven't already, just so he's not so vulnerable to disease. Call around at your local animal rescue organizations and you might be able to find a place which will help you with the initial vet costs. You can just trap him, get him checked up, and release him. OR you can work with a local rescue organization to help him find a new home if you really want to get him off the street but worry about expenses.