Have you spoken to Fidelity? I suspect this is far easier than it may seem, right now. In 30+ years of dealing with Fidelity, I've yet to speak with a representative who did not know how to get things done. This seems like an easy one; I'd be surprised if one phone call didn't get you the right, and easy answer.

Perhaps I'm oversimplifying, but why wouldn't you (or your parents) just write a letter, instructing Fidelity to deposit the enclosed check into the following account: your son's name, account number, etc. Or better yet, take the check to a local Fidelity office, if there is one near you, and speak to a human being, and give him/her the instructions. Perhaps even with a letter from your parents in hand.