Sorry to hear this. His denial may be a subconscious way of thinking if he doesn’t seek treatment nothing bad will happen. Other than education there may be little that you can do while he feels OK. If his next headache is severe consider calling an ambulance to take him to the hospital then and there. A rupture could lead to stroke, brain damage or worse. I wish you both the best.
http://www.brainaneurysm.com/index.html