You could always grant him a piece, say 5%, with incentives that can be granted over time while certain company KPI's are met (you both could co-write these), pushing up closer to 10% in the event of some sort of fund raising he would be a part of. This covers a lot of your risk, and sets the stage for a solid working relationship. You need to manage both in this instance, which makes the relationship more complex, but could end up with you having your cake and eating it too.