how do i politely tell my boyfriend that he needs to eat healthier and lose weight??

so me and my boyfriend have been dating for almost three years now. he is so good looking but he has gained about 70lbs of pure fat since we started dating. he eats so bad and doesn’t exercise at all. it’s a super sensitive subject with him and I don’t know how to make him motivated to lose weight especially for his health. I am a very health conscious girl and exercise all the time so it’s hard for me to understand. help!!