Once consult doctor

Feeding RC to your pup doesn't mean he will gain weight or become healthy. RC food is for the overall development of the pup not for weight gain It's good that he doesn't have obesity. He is a pup with lots of energy, which makes him fit

Focus on growth and nutrition to your dog, he is just a pup. When he becomes an adult he will have a ideal wieght.....i don't think being skinny is a problem Even human adults are skinny that doesn't been he is weak....

And even if your problem is not solved. Then consult a Vet for better consultation

Dont go for weight gain go for activeness in puppy and give him deworming then he going to gain weight