How much stability are you asking for him to sacrifice in order for the deal? ownership of the company should be thought of in cash terms, it can then be traded

How much stability are you asking for him to sacrifice in order for the deal? ownership of the company should be thought of in cash terms, it can then be traded off directly between salary and equity. Give him a generous full package of what you honestly think he's worth, keeping in mind his current position + stability of that position vs potential stability of the position in the new organisation, and then let him pick out the details of how he wants to split it between investment in the company and raw salary. The results should be pretty illuminating into what he really thinks of your venture.