Relationship weight

Me and my boyfriend have been in a loving and health relationship for a little over a year now. He is the kindest, sweetest guy. I have always been naturally very small with a high metabolism. When we first started talking he had a very athletic build due to being on his high school football team a couple of years back. But within the past couple of months he has put on some weight and he makes comments about it to me. It doesn’t bother me BUT I do want him to be healthy and take care of himself. How can I go about encouraging him without sounding degrading or making him upset?