He's gorgeous. The flub you referred to, that's his skirt! Like jowels, males tend to get skirts when they start putting on excess weight. Some females do this too, but I've seen it more in males. I'm assuming you've had or will have him altered - that's going to help him develop a really nice skirt, lol. Its nothing to worry about, but since he's obviously fine weight-wise, I'd start mixing in adult food and getting rid of the kitten food.
And tell your daughter not to make too much fun of him, its going to hurt his feelings. One of my boys has a skirt and his owner called me the other to let me know how they were doing. They make fun of his skirt and he knows it and it hurts his feelings and he goes and hides. LMAO.