need tips for son to gain weight

My oldest son which 7 years old is much skinner compare to other boys same with his age.. his weight now only 14kg.. can i give him scott emulsion for him just to gain weight? for your info, his appetit to food are much more bigger than i am but he is not gaining any weight. I also gave him pediasure milk but he is getting taller, not gain any weight.. please help..