If you did not have a clear, written vesting agreement, you should contact your quitting co-founder now and align expectations, then get them in writing.
You should do that whithin days because, now that he's quitting, there's a nonzero chance he'll just sign off any rights he has to you, for free. Just say you need to give out the equity to replace him, so this needs to be official. Hopefully, you parted on good terms.
Sorry to add to your worry pile, but if that remains unclear then a couple years down the road, when you're about to exit, he could show up still expecting to own half the company.
You should do that whithin days because, now that he's quitting, there's a nonzero chance he'll just sign off any rights he has to you, for free. Just say you need to give out the equity to replace him, so this needs to be official. Hopefully, you parted on good terms.
Sorry to add to your worry pile, but if that remains unclear then a couple years down the road, when you're about to exit, he could show up still expecting to own half the company.
Good luck!