Is he still going on about it? Just don't meet up with him when he's got the bike. Round a carpal at slow speed is worse than doing 40mph as it takes a lot more skill to ride and corner at slow speed as the bike is less stable.
Would he be happy to just find a group ride out near you. Or go to your local bike cafe and talk crap with other bikers for a few hours.
You could pay for a refresher course for him for his birthday.
I'd also ring the hire company and ask if they allow minors as pillion as their insurance would probably not cover children. Also do they make him do any kind of test to see if he can actually ride it before letting him take off?
I ride a big bike - BMW R1150GS - both off and on road and it has taken many years of riding to become skilled.
I have ridden hundreds of thousands of miles but hardly at all in the last 2 years. I have just declined an evening ride to London with DP to see tower bridge illuminated because I know that being rusty I will not be able to safely filter through the traffic - and I know what I am doing.
The fact that he hasn't even thought about full protective gear for DS is madness.
Anyone who says they will not have an accident is a complete twat. He may not be the cause but it only takes one person "not seeing" him and pulling out of a junction and with his inexperience he will be unable to take avoiding action.
If you really can't face another row just take the kids somewhere else. Get up tomorrow morning, pack overnight bags and go stay with someone.
If he really doesn't see your point of view then maybe you need to consider your future with him at some point.
But take heart he cannot forcibly make your children get on the back and if you don't meet up with him there will bee no opportunity,
Good luck. Have been thinking of you and how hard you must be finding it. You are doing the right thing. And I say this as someone who will take DD as a pillion when she is old enough and if she wants to, and when I have recovered my skill.