My son is 3yrs old and has a bad rash on his butt and the back of his thighs is their something over-the-counter i can buy or should i take him to a dermatologist?

Without seeing : The rash , it is difficult to say. Most of the time it is from contact with something , or sweating or detergent from his clothings. He may have sensitive skin . Best to take him to your pediatrician and go from there. Refer to dermatologist only if necessary.Aquaphor healing ointment is pretty good . He may need strong steroid rx if inflamed . Otherwise u may use Cortaid (hydrocortisone) 10 over the counter.