I spoke with Anne, the lady who took care of him and she told me that he is happy this way, that the extra fat will help him go through winter and that he won't like to be an inside cat. She told me he threw a fit when they took him to the vet.
But he seems so friendly and trusting that I can't believe he won't like to be in a home.
A cat outdoors is not the same as a cat indoors. Some cats have never been inside or don't remember their days as an indoor cat, they will feel confined and stressed indoors. Blacky is a pretty good example of that. Once I'd tamed her outdoors I tried bring her inside and the first thing she did was flip out and jump out a broken window that was way up high, one that I'd never even considered she would be able to reach. It took a long time for her to warm up to the idea of being indoors, and that only was accomplished by having a cat door where she learned to slowly trust coming inside with the stipulation that she could always get back out when she wanted, 24/7. Even now 10+ years later, she hates
being shut inside. She is entirely comfortable in the house, but don't you dare close that cat door on her. She'd rather be outdoors and doesn't want to feel trapped, even if being "trapped" is the entire two story house.
I'm not saying every street cat is like this, many adjust well to indoor life but this cat may be one of those cases that doesn't "get" the indoor lifestyle.
The cat could have been highly stressed at the vets... so that is not the best location for deciding if the cat is a good indoor candidate; Jasper being taken to the vets was done in a humane animal trap. He sounded like a wild animal in there (in all fairness, at the time he was semi-feral) but under 6 months later he'd become the is the most laid back house cat there is.
My suggestion would be to lure him indoors and see his reaction when you close the door. If he flips out you'll know at the very least it will take some time to get him used to the indoors. If you're not going to be the person to do that for him, then I'd leave him as the happy outdoor cat he is. However, I agree with Marie on the weight issue... something should be done there. Find out who is feeding him, or pool your money together to get him some good food and feed him at one location.