HELP.

Hi mommies I have a quick question to ask.. my little boy is 7 years old and is giving me a hard time with eating! He doesn't eat, nor does he  finish all his food.. I've taken him to the dr .. dr tells me to give him shakes which he refuses to take! His father is about 115 pounds so it's safe to say the skinniness comes from him lol😒 but I really really don't like it .. any recommendations on vitamins ?