I know this is a very painful time still my understanding is that Marriott only have one preferred water sports vendor and it is called Red Sail Sports, I was told it was because they were the only one who have insurance...
When you say purchased in the Surf Club property do you mean inside the lobby or by the beach behind the Surf Club?
As for the Police I don't know the laws in Aruba but was there an official report? Is it illegal to drive when underage? Is it required by law to be licensed? (Common sense and law sometimes differ...)
I've you try to contact the vendor for a refund of your medical expense or ask if there were insured? US medical bills and penalties probably won't be refunded unless you hire a lawyer in Aruba or if you were already insured for such risks then your insurance company will probably seek refund for themselves....
All these formalities are very painful still the paramount thing is that your son recover fully of this horrible accident.