My daughter was constantly active, ate more healthily (in general)than her peers yet was in the upper range of average/abit tubby this stayed the same even after she stopped dsancing/swimming/singing & sport between ages 10 & 12. Then at 14 she got braces/hit puberty & it all changed, in fact she now needs to gain some weight to be really healthy.
It does depend just how overweight your son is & whether it's him who's affected by it or you who's affcting him with unnrcessary worry???? Just give that possibility some thought.
Also though do look at what he eats & when, we are all unique & maybe he needs different foods, or to have his meals at differnet times or larger meals at different times or 5 small rather than 3 large meals a day?? Also he does need to be active, preferably in a sport that he likes & is good at, which would also help his self esteem.