Do his part... that is life unfortunately. It won't be the last time you are stuck with someone who is lazy, and the work still must get done. The best solution is to do the work, tell the guy you aren't impressed with him, and either leave it there or tell your teacher what happened. It isn't really ratting... or you could just say "listen, I don't want to be in his group again" and they'll get the idea.
It sucks being in that position, I've been there. But if you have to take his load to do well yourself, then you have to suck it up. If you take the chance on waiting for him to do it, you may end up screwing yourself and your whole group over.