I'd be willing to bet that he already knows the dangers he faces. He probably has already done some research on it and the diet scares him more than the possible consequences. After all, the complications are "could be's" but the diet is "must be". Ot's kind of like a smoker - we (I am a smoker) all know about the lung cancer, etc. that we might face. But quitting is HARD, and it's easier to just not think about the future POSSIBLE consequences.
So what I whould do if I were you is concentrate on the positives of the gluten-free diet. Talk about how GOOD people who go gluten-free feel. And maybe make him (if you are a good cook) a gluten-free meal. Even if you don't live in the same area, you might be able to send him something. Maybe some Udi's Double Chocolate Muffins - those things taste BETTER than their glutenous counterparts. If he knew that he could still eat delicious food, maybe he'd be more willing to try.
Where does he live? If he is in a city, odds are there are plenty of places he can get gluten-free foods, both to cook at home, and at restaurants. Help him do some research on that, and give him as much support as you can.
BUT, if he doesn't get on board there is nothing more you can do. We all have to make our own choices. I think a lot of us here have friends or family members whom we know have celiac and refuse to stick to the gluten-free diet. We bleed for them, but we can't force them, just like we can't force our smoking friends to quit.