Angel of Faith - Your Boyfriend is Lucky.. There are many who wish they had his problem.......
BUT, I think you can fix that. Over the next month or two make sure he starts eating lots of Cheeses, Nuts, and especially carbohydrates, like potatoes, bread, pasta, and the like (butter on the potatoes and bread, cheese and tomato sauce on the pasta). Dont take his word for it... Watch him eat... For good measure add in a few beers or drinks at dinner and maybe some cake and or ice cream for dessert.
Then there will be no way that he (or you) will have this problem anymore.