He's not a founder, he's an advisor. Advisors at that stage typically are granted 0.25% - 1% depending on what's expected of them. More than that would raise eyebrows. He is not a founder though.
Talk to him again and talk this through. If you can have that conversation, which is hard, and adjust his equity to a reasonable amount, then your likely good of success goes up dramatically, partially because you fixed a real problem, but more importantly because you were able to do something hard.
At the minimum, their equity is under a vesting plan?
If they believe in this company and want to help it succeed, they can invest in it with a safe, which gets them equity rights and a strong incentive to help you succeed.
They take zero risk, they get no equity.