[sajid]

If you're on friendly terms with your cofounder then it's just a question of reaching an agreement both sides are happy with. The best option would be along the lines of what davidu has suggested.

If you're not on friendly terms then it's basically irrelevant that he didn't contribute anything to your current success. There's nothing you can do without his consent.

A 1 year cliff is standard, and he stayed with the company for longer than a year. And since you had a vesting schedule in place, the cofounder is clearly entitled to the stock he's accrued.

It seems to me that the mistake you've made is having an overly generous vesting schedule. 4 years is standard, in which case he would have only accrued about 8%.