In Stewie accent
Hey fat ass, you're a fattie. You should probably do something about that, huh? Fattie, huh? You've been eating a lot there big guy? A little bit of butter on your bread there? Hmmmmm. A little bit of extra cheese? Hmmmmm. A little bit of super size me? Hmmm.
End accent
But seriously, if he's catching an attitude, he probably sensitive about the subject. Approach him when no one else is around so that he doesn't feel attacked. If you're sincere and not joking around, he should perceive you as so and give you an honest response. Offer to work out with him or something. Give him some tips on eating better, etc. Be supportive.