Well... he looks quite well and handsome to me, but you're his mama -- if you feel he's getting thinner, then he probably is.
You say you can't handle a vet visit right now, but... have Tom and his sister been going to the vet regularly, getting their shots and boosters? Those are especially vital for kitties that are allowed outdoors, where they're exposed to all kinds of dangers that could account for weight loss.
Here's a thought that may or may not be valid: I know male cats are normally a little thicker through the neck and shoulders than females, but neutering keeps them from developing that upper-body heaviness... so maybe if Tom had started to develop that typically masculine proportion and now, after neutering, is losing it, that might give you the impression that he's getting thinner. How old is he, and how long since the surgery?