My Stepson Is 4'11" And 175 Pounds. What Should He Weigh?

First of all he should only weight about 100 pounds if between the ages 10-12, but if he is older such as 13 14 15 he should weight between 130/135.

If your Stepson is a good listener, have him aim at protein to keep the appetite at bay.  It's a good muscle builder too.  In a pinch there's a few good bars out there he can munch on.  Look for the best tasting ones at a bariatric center.  Sure it's a bummer to go there but believe me, they have the best of the bunch. For example the Rock-in-Roll is similar to a Payday bar. (Of course you can't be allergic to peanuts.)  I was a sugar-hound and thanks to an Internal Medicine doctor was able to kick the habit.  My other-half who loved Cadbury bars followed my advice and stayed off sugar items completely for an entire year....and of course the benefits became obvious--just in cavities alone.  You can tell him I've narrowly escaped surgery because I knuckled down just in time...and escaped diabetes in the process.  Good Luck

I suggest you get into walking with him, he isn't going to want to do this by himself, and at the same time it is going to be good for you two to bond even better. You want to make it a brisk walk, for about a 1/2 hour to start, and do this several days a week if you can. Make sure that he is eating healthy, fruit and salad is great, and trim the fat from meats, this is far better for him than a meal out of a box, and a treat now and then isn't out of the equation, just keep them for rewards for a while.
After a week or two, you can step up the amount of time that you are walking together, and who knows, he might take the initiative to walk on his own too, or take up biking. Being physical is going to be key in helping him to lose weight. Hope this helps, best of luck to you.