Do you have a vesting period? Pretty sure you didn't, but just making sure.
This wasn't because of lack of enthusiasm, just an economic reality for the guy.
But wow, as soon as we kicked off, it was like Jekyll/Hyde. And I've known/worked with this fellow for ~6 years, in fact I escorted him as first hire into his last gig where he did an outstanding job for several years.
Founders should always have vesting. It is going to permanently screw up your company if this guy has some large percentage of the company and isn't a part of its success. Plus the rest of you should have vesting anyway in case another founder doesn't work out for whatever reason. Everyone should ALWAYS have vesting.
So my advice is that, if you haven't yet signed the papers, you should go back and put vesting on all of the founder's shares, including his, of course.