Getting lightheaded

My boyfriend has never had this problem, but he’s been having a problem recently when he gets erect. Every-time he gets hard for longer than 15ish minutes, he’s been saying he gets lightheaded and gets headaches. I think I’m a he’s a healthy person as he exercises regularly and eats well. What can this be and should I be concerned? I don’t want to sacrifice our sex life unless absolutely necessary, but this is really concerning me. Do other guys have this as well?? For reference he’s about 9 inches and he’s joked about “all the blood going to his dick” but I seem more concerned than him. Any advice would be greatly appreciated