I was worried about it. I had more or less given up. Had sent a query to the club, nothing back for a week. Last night I was in the very process of e-mailing a friend that I didn't think I'd be able to do the ticket transfer to him (he's on Friends and Family on my account; an e-mail came through from the club talking me through it step by step. It worked like a charm, he e-mailed me to say that he'd got it on his mobile (he's on Android, so he'd previously downloaded the Google Pay app — it's in there). You should have got an e-mail from the club. Don't despair. You might get one today, or tomorrow. I know, I know, they're cutting it fine, I had given up hope, but the system does work. Give supporter services a ring if you don't get anything.
Your case may be a bit different, in that it's your son's ticket that you're trying to transfer. But the system does appear to work, albeit creakily.
I think people have to realise that this is a big, big deal, involving thousands of people. I've got no idea how many out of all seasoncard holders take advantage of the Friends and Family option (and I also wonder if all other clubs offer it, tbh), but it'll take some ironing out, for sure.