If he's blacking out, it is definately a problem. He could be dehydrating himself - that's my first thought. I don't know much what you can do besides express your concern to him. It sounds like you've done that already. Possibly recruit some of his friends to encourage him to be more careful. They will have his best interest at heart and should be glad to approach him about this before he hospitalizes himself.
Also you could look up some reputable rescources on the subject and show him that you aren't just overreacting. One rescource could be
http://my.webmd.com/hw/health_guide_...navbar=aa72754
Another thought is if you are every with him and he faints - immediately call an ambulance. When they show up, even if he's recovered already then he's forced to explain what is happening and they can tell him what harm he's doing to himself. Maybe it will be the wakeup call he needs.
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"Always learn the rules so that you can break them properly." Dalai Lama
My Karma just ran over your Dogma.
