I definitely think that you should keep him as an indoor cat. The lifespan of an outdoor cat is extremely short.
As for getting another cat, I know that it would give him a playmate, but on the other hand you have to think of unexpected vet bills if your animal were to ever get sick, so if you are having trouble affording 1 cat, I wouldn't bring in another.
As for the biting, my cat used to do the same. I stopped playing with him with my hands and feet and ignored him completely if he tried to bite. I played with him a lot though until he got tired out which really seemed to help. He has since grown out of the problem completely.
Lots of attention and playtime should stop the problem.