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I'd say he's good for now, he's got a great card with an excellent SL, no need to go crazy with apps or anything. For optimal scoring he would need 3 cards and no more, but if he is currently comfortable with Amex then leave it at that, otherwise I'd look at a discover and a chase freedom for the rotating catergories or look for a card that covers dining out if he does that a lot, otherwise the BCE is a well rounded card that will serve him well, Congrats to him on his Amex approval.
There's really no advice that I have that you don't already know from managing your own credit. For him I'd say have a couple cards, pay in full every month, rinse and repeat. This builds a credit history. With a 20k income, I doubt he is spending enough to justify several rewards cards, so I would say the Amex and possibly one other card (a Visa/MC) should be enough for now. There's no need to go crazy with apps because the income just isn't going to create a lot of spend/rewards across several programs.
As a side note, IMO, don't let him get in the habit of thinking to just pay over the minimum. Pay in full at all possible times. Especially for a young person it can be dangerous to start falling into the mindset of "just paying the minimum" because things can rack up quickly. There's no reason to be paying interest on ordinary, everyday purchases.
Total CL: $321.7k | UTL: 2% | AAoA: 7.0yrs | Baddies: 0 | Other: Lease, Loan, *No Mortgage, All Inq's from Jun '20 Car Shopping |