I waited a day after seeing this thread to post, and thought about not posting at all, but here goes:
First, Santa could have had more tact, but someone needs to say it. The mother may or may not have tried to instill good eating habits, but perhaps she doesn't know what those are herself.
Now, on to the meat of the post: I am the father of an overweight kid. Part of it is he didn't get great genes from his mother and father. A lot of his friends eat and eat and eat, and are skinny, with not a lot of difference in physical activity. Sorry kid, you got dealt a bad hand there. He also seems to have inherited our sweet tooths (teeth?). And yeah, if you saw us out in public, especially if we were enjoying a burger and fries, you'd probably look at us a lot like you look at that kid and his mother.
Don't assume that healthy eating and an active lifestyle haven't been drilled into him. Don't think for a minute that I don't tell him every chance I get about good and bad eating, and about how much easier life is when you're not fat. I know how (*&^$% hard life is when you are fat. I'm down about 90 pounds from my peak, and I've still got a good 30 to go. But Mikie's post above is right - mountain biking saved my life, literally, along with learning about better nutrition and better eating habits. Healthy eating is still my weakness. I like my beer. I have a sweet tooth. I don't eat enough vegetables. But I'm a hell of a lot better than I used to be.
And I'm doing everything I can to keep him from going down the same road I traveled. I talk about nutrition every chance I get. I talk about being active. He gets out on the bike with me (but not enough), he goes to the gym with me. His baseball hitting coach has tried to instill the importance of being fit. I'm trying not to be a food Nazi, and try to tell him it's OK to enjoy a burger or pizza now and then, but eat sensibly the rest of the time. Slowly, some of it seems to be taking. He still isn't likely to eat much in the way of vegetables. He loves bread and potatoes. He won't go near fish. But I am seeing him not finish everything on the plate at every meal, which is a big improvement. I believe he's seen results from the gym time, and that's encouraging to him. So there's some hope. And he's 12. Even at 5'6" - yeah, he's a big kid, going to be several inches taller than me - he's got a growth spurt coming. So he may very well grow into his weight.
Believe me, it's a damn fine line his mother and I walk. There are days where I want to get rid of the XBox and TV altogether, force him to ride with me, serve fish and vegetables for dinner and tell him if he doesn't like it, he can go to bed hungry. That's more likely to be counterproductive, and lead to him sneaking food, the worst kinds of food. It's kind of like an alcoholic - you can't force an alcoholic to stop drinking, he has to want to stop. Thus the education. The positive reinforcement. The encouragement to be active.
My biggest hope is that we're setting a decent example. His mother plays tennis and will occasionally ride a bike with us. I bike, run, swim and train. We're working on setting better examples in our eating.
Not sure what the point is, but I know this parenting stuff isn't easy. Sometimes you don't know the right action. Sometimes you do everything right and stuff still goes wrong. And every once in a while you do things right and get the expected result. Those are the days that it's all worthwhile.
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I wish you the absolute BEST, and I know we are all with you!