All he does is sit on his chair and watch TV is correct. It means he does nothing else but sit and watch TV. I would say that sit and do are bare infinitives [without to] and in this construction an infinitive is required not a Gerund or present participle. One more thing: in a chair or on a chair. I would probably expect in a chair here because he is watching television and he is probably sitting in an armchair. However, if, indeed, he is sitting on a wooden chair at a table, then on a chair would definitely be correct.

does - sit is doing - is sitting So you keep the patterns consistent: - All he does is sit in his chair and watch TV. - All he's doing is sitting in his chair and watching TV.

All he does is sitting on his chair and watching TV. "is" is a verb and "sit" is a verb too.There can`t have two predicate verbs in a sentence.So I think "sit" should be changed into sitting.

I'm really impressed. I knew this site was helpful. Now I see it's the location of such good friends :) Thanks everybody.