Any conversation should be phased as “I want to progress – what training can I do?” Not how much will I get. Look like a contributor not a taker.
But being prepared is a key to making it a worthwhile conversation for basing any future decisions on. Do the CV. That will help get a real handle on your son’s skills, for agencies to suggest alternatives, for the current employer to realise what they might lose, to identify possible training opportunities. It will also help your son start or reinforce the right terminology for any conversation.
The suggestion to look at the company structure and pay scales is a good one. It will help decide if the prospects are reasonable, if the gap is reasonable/achievable, if qualifications are really a barrier.
Doing any qualification/training is never a waste. It gives you time to learn something and get a piece of paper that will open doors at the very least or it gives you time to reflect on things (crap or otherwise)