If he owns > 50% he can dilute you to whatever he wants. But, if he owns less than 50%, then he'd need to get voting control to do it. Is that possible? Do you own enough to prevent it? If so, I'd just tell him you're not interested.

If he controls the Board, then he can do pretty much anything. He can create more shares and give them to himself (that would possibly create tax issues, so he might not want to do that).

Also, I'm curious how the stock is valued. Do you know that?