He's probably a director of the company and not an employee. That tends to give him additional rights (and responsibilities) that you don't have.
One of those is that you usually can't "fire" a director without board approval. Some director agreements and vesting schedules (if drafted properly) also have special acceleration clauses on termination without cause, so even if you could get rid of him, he'll still get his part of the business.
tl;dr: I don't think you can make that decision - it's up to the co founders and the investors. There's nothing you can do, and even if you could, he'd probably leave with his fully vested equity stake. Time to find a new job if you can't deal with that.
One of those is that you usually can't "fire" a director without board approval. Some director agreements and vesting schedules (if drafted properly) also have special acceleration clauses on termination without cause, so even if you could get rid of him, he'll still get his part of the business.
tl;dr: I don't think you can make that decision - it's up to the co founders and the investors. There's nothing you can do, and even if you could, he'd probably leave with his fully vested equity stake. Time to find a new job if you can't deal with that.