I was once in that same situation as is your son

I was 20 yrs old and gullable when I was shown the plan. What got me out was my sponsor who I respected saw something screwed up about our upline emerald(now diamond). He thankfully broke the cardinal rule and passed negative downline to me. I am indeed thankfull that he was a “negative loeser”. My parents throughout this whole thing remained supportive and non-judgemental,even though I’m sure that they were happy when I got out.
As far as what you can do. Let him know that you love him and you support him no matter what he chooses to do. The way the AMO systems work is in that it makes them think that outside resistance is a sign to press harder and those who oppose what you do, no matter who they are, are to be avoided and/or ignored. He must see the obvious for himself nobody can point it out to him.
I hope this helps