I would agree that you shouldn't try to stop him form doing it if it's important to him. You don't actually have to "support" him. Just kind of disengage from it, if possible. Give him the space to do it. Ask him to get some good life insurance. If my husband wanted to take up motorcycles, I'd tell him, fine, but double your life insurance.
Get a motorcycle and ride with him. Might be really fun.