I believe talk to a lawyer for your options. You may give him some non-voting percentage. You can put some cap on valuation. e.g. he gets 10% stake. and you tak

I believe talk to a lawyer for your options. You may give him some non-voting percentage. You can put some cap on valuation. e.g. he gets 10% stake. and you take company to 100x, his stake may not grow equally. You can have an option to buy his shares at some calculated valuation in some future funding rounds...