What can i give my kitten to gain weight?

OK so i got this kitten bout 3 weeks ago and he was to start but with so we started feeding him kitten chow wet and dry and i was fixing my other cat her food and he went running right over to the bowl and ate some of it now all he eats is that he wont eat his kitten food at all and me and my mom are worried he not getting enough nutrition hes about 3 month old Maine Coon what should i feed him if he wont eat his kitten chow wet or dry is there any this cheap at like pet-co or anywhere hes already been to the vet and he had no worms or intestinal problems can anyone help me please