My pup is about 9 weeks and has a thick upper body and rather scrawny waist to hind legs, is this normal?

he is either a gsd or gsd mix and he is at 9 3/4 lbs @ 9 - 10 weeks(the vet estimated) he recently had a serious flea allergy and was was losing hair all over his body and especially his face im wondering if the flea allergies could have caused imprpper development or weight loss or simply just let him not gain weight, i feed him puppy chow thrice a day, is this sufficient enough for him?