To all you asshats that say he just needs to eat less, exercise more, etc. Life ain't that simple.
My friend's youngest brother was also shorter and thicker than all his brothers. He ran, ate healthy and did all the other things, but the weight didn't come off.
Until he was around 14 and a real life isekai god took pity on him. Within 2 years he grew taller than his bros, and gained enough muscle mass to destroy everyone who'd picked on him. Never increased his workout routine. His body just decided to change.
Two lessons here. First, don't pick on the short fat kids. Second, the human body is complex. Don't think "by just doing this" that something will change.