Neither one comes off as rude, both have some validity to them and either one might be preferable depending on the situation! But I think what would be even better is asking him how he wants you to respond when he stutters. There are some people that I'm okay with "helping" me with what I'm trying to say, and maybe you fall into that category for him (or will eventually, with time). The most important thing is him knowing that you're comfortable talking to him, but letting him tell you how best to communicate.