They are both correct, except that 我不能帮他＝I'm unable to help him. The word "able" means to have the ability to do sth. So... it is not proper to translate 不能 to unable. 我不能帮他。I can't help him. (For example, I'm able to help him bu I can't. Why? It's better for him. People need to search solutions themselves. ... You know, what I mean :-) ) And, as 会 has two meanings, 我不会帮他 has two meanings. The first one is what 'samantics' said. The other one is that I would not help him. As additional information I would say: (这件事)我是帮不了的. Simplified for spoken language, you know.

我帮不了的 ［Wo bang bu liao de] meaning: (This is) not something I can help with. 不了 [bùliǎo] is used after a verb that means cannot; be unable to; be impossible to. 我帮不了他 ＝I'm unable to help him/ it's impossible for me to help him/ there's nothing I can do that would help him. 我不会帮他＝I don't know how to help him/ (I refuse to help him). 我不能帮他＝I'm unable to help him/ I can't help him (but it's not necessary impossible).

= 不了

Thanks to all friends for your help and explanations! ^__^