Its difficult knowing what to say to the kids as they are all at different ages, maturity levels etc. I was honest with my eldest daughter who was 12 when I had surgery, but with my youngest who was 5 at the time I just told her mommy had to have an operation on her tummy without telling her any of the risks.
If he is "mouthing off" then I would actually take that as a good sign - at least he cares enough to show some emotion. If he has some specific questions, post them on here for him and show him the answers you get. Get him to look at Bospa site which is very factual and contains some stories, and importantly tell him what health problems you face now and in the future if you dont get your weight under control. The risk of you dying from obesity related disease is higher than the risk of you dying on the operating table. Also, see if you can find yoru surgeons stats re complications and mortality rates.