I would have taught him all about fat people and sat on his scrawny little butt until he begged me to stop.
I wonder how people would feel about this same situation if he said it to dad and dad cracked him in the mouth? Odd how it's more acceptable for men to be fat than women.
I realize my being fat might be uncomfortable for my daughter but guess what, lots of things in her life will be uncomfortable and she'll have to learn to deal. There are plenty of things she has done and will do that will embarass the heck out of me, but you know what, I'll learn to deal because I'm her mother and love her NO MATTER WHAT. That's something that kids need to learn about love -- it's unconditional. You can't let them think it's only okay to love their spouse if their spouse looks like a trophy wife/husband. Then what, life happens and it's okay to discard your spouse for something better looking and more well maintained? How about a child born with a deformity -- lock it away somewhere so your shining example of the perfect family isn't tarnished?
Fat is the last acceptable prejudice.