You write down your point of view and ask what you really want. Start from here.
Hey, we are friends, but enough is enough. I feel the current situation is unfair. I work full time and I should get something like a salary but not you, because you are not full time.
If you cannot find a consensus, then, may be you should ask a lawer to know what is considered fair by law.
If you have so much to get out of your chest, you should may ask the help of a mediator.
But in the end, there shouldn't be something you didn't tell him. Be clear about what you expected and how much you were disappointed. And also be clear about your point of view about him going full time.
If you can't find a mediator, the best way of telling him is by writing a list of facts. Try your best not to use judgment value. Just facts, then what you want and what you don't want.
If I understand correctly, your ideal situation would be for him to quit the startup for free. But let him explain his point of view. After a long discussion you might find a solution.