Can't pet without getting "excited"

So I have a special needs cat named Billy, who is missing an eye and only has like 25% vision in the other eye. He's so loving and sweet and he seems to know that I rescued him, since he's always trying to find a way on my lap or in my room. I'm having this weird issue though, and maybe someone can shed some light on it. Now that he's a little older, I can't pet him without him getting "excited" if you know what I mean. He isn't fixed, since he's an inside cat and isn't ever around females. He's never sprayed or anything, so I didn't think that spending the $100 would be worth it, but now I'm wondering if I need to look into it again. Anyone else ever have this problem?