Hi Amit, I give me son half a tablet, but he's only 4 and weighs just about 30lb. I would think that in comparison to your sons greater age and heavier weight, a full tab would be fine. My son has been on half a Protandim (and 300mg Idebenone) for well over a year - with no noticable side-effects (apart from sometimes darker or more 'yellow' urine).
Be advised one of the side effects of Protandim is abdominal/gastrointestinal issues ... we used 1/2 tab for our son Sam (45+ pounds) for about 1 1/2 years. He developed pretty severe GI symptoms after that time, and through trial, error, and success figured out he had developed lactose intolerance and had developed an intolerance to Protandim. The G.I. symptoms are now absent!
My son is 35kg ; will be 9 in Aug. I give him a tablet per day. I do not see any side-effects.