What Causes Low Appetite In A Child?

Hello Doctor !! My son is 2.3 Yrs old. He is very active but most of time he is not eating well properly, & his weight is alos not gaining so our doctor started him to give CYP -L. after given him CYP-L my wife observed that my son is sleeping too much time compare to normal time. So shall we are confused to stop CYP-L or continue??