In that case, my suspicion is that even the threat of a buyout isn't going to do any good in terms of motivating him. He really needs to go do something else,

In that case, my suspicion is that even the threat of a buyout isn't going to do any good in terms of motivating him. He really needs to go do something else, at least temporarily, to resolve whatever's going on in his life. Having him stay on helps neither of you, and as a friend and co-founder, he should recognize that. When it comes to dealing with his share of the company, I can't claim to know how best to deal with these matters, but if he's made a significant contribution to the project overall, perhaps he could retain a reduced share in exchange for leaving the project early. (Someone tell me if this is a bad idea.) As far as how to move forward, what will it take to complete the last 5%, and how tight is your current situation?