I think size makes a difference too… my endo has some ratio xu/day/kilo or something (I am not sure of the exact ratio is exact, but just to give you the picture. Your 6’2" son should probably need more than those of us who wouldn’t be 6’2’ even if we were on stilts.
Hahaha, that would be me…barely 5’2"! 
I will look in my books about the weight thing.