This is a known issue when you pay off longer term loans. Your credit score is looking at the mix of debts that you have, how well you pay them off, etc. Now that the DVC loan, car loan, and closed card aren't in the mix, there's less for the system to take into account when giving that score. It's counterintuitive, but true, that paying off big loans hurts your credit temporarily. In the future, if a card gets hacked, have them just send you a new card number, don't close it outright, unless you really no longer want that card. That would have helped his current dip somewhat, but not totally given the two other paid off loans.
For the United card, yes, it would give him more in his credit mix, but it would also be two applications in a really short window, right? So the benefit of having the card may outweigh the cost of new credit being opened so close together. Just having the CSR open for a while and having good payment history on that is going to help bump things back up.
Is there a loan you intend to open in the near future where his credit score is of particular concern right now? Because if there's not, I'd ride on the CSR and give it some time to rebound. Do the United card in your name as planned, then maybe do the next one in his when you're at the next SUB.