All my questions are for your consideration, no answers to me are expected.
Different room, reservation = separate booking windows for excursions and PATs.
That is something to take into account. Are you willing to board later to accommodate his assigned time? Or use a
DCL transfer that makes PAT a moot point?
If he is booking solo, had you considered making that room assigned to one of the adults in your family with him? If he paid you the difference between your son’s 3rd person fare (with you) and the cost of the single supplement, the friend would come away with a substantial savings. This would also put both cabins at the same CC level (if you had 2 family members at Platinum). This could also alleviate PAT differences.
If he is capable of creating a DCL account, great. Otherwise, offer to help. And provide him with dates related to booking, check-in,…
And possibly have a chat concerning what responsibilities you have for him and expected behavior (any of the kids). This includes his adults.
Verify he has passport or adequate ID.
It wouldn’t hurt for your own kid to know how to book and manage a cruise reservation, while you’re at it. Sometimes even couples have a person unaware of the procedures.
Make sure he has same dining time. I don’t think you have to link reservations until closer to cruise.
If he is going to use his dad’s credit card, he can put that in during online check-in. Or, as mentioned, select “cash” and add the CC info later.