My boyfriend says he is sick when he has hayfever!
People have different opinions on what 'sick' is.
There isn't any chance of dying younger purely because you have celiac unless you carried on eating masses amount of gluten then you might run into some complications later on. But you are being so careful.
I'd just get it out into the open and explain that having an auto immune disease (which is what arthritis is too btw...) doesn't mean you will die young or anything or the sort, people who are morbidly obese will die younger... hint hint.
He needs some knowledge and you are the best person to give it to him.