Only you know your child, but at age 15, I would have had no problems letting my boys fly on their own and board DME on their own. They had been to MCO many times by then and knew how to read signs and get where they needed to be. My boys were also over 6 ft tall by then, so there would have probably been no questions by the DME folks on their age.
If your child appears confident and acts like he knows what he's doing then he'll be fine. However, if he tends to be timid and would act unsure in this situation, it might be better for you to plan alternate transportation.
If your child does get questioned by DME about his age upon arrival and denied boarding, then instruct him to ask the DME agent about a taxi and then pay for it upon arrival at the resort.