I would assume that using signlanguage he could communicate as anyone else using signs. I am by no means an expert; so take it with a bucket of salt. My opinion and, the swedish medicine branch opinion, is that (afriad I dont know any better words for it) 'avoidance/escapism' is the real issue here. You should not be judged or inconvenianced by how you inherently speak; and softvoiced nor stammer should be the lone cause for learning sign. Id recommend talking about it, in a light tone :) aswell as when he stammers you dont fuss about it; and he'll eventually be more comfortable stammering infront of you.