I had to look up what it was. Honda RC51 RVT1000R. Yes, that's way too much bike for a 16 year old. Even if he's been riding dirt for years and rides street now, a liter bike is close to a death sentence for a 16 year old. It's yours, not his. You're the Dad. The word you're looking for is "NO". Headstrong is another way of saying he's not going to do what he's told. That doesn't go well with a fast bike. Tickets and attempting to elude quickly follow.
So put it up for sale w/o telling the kid or Ex. When it sells, you could offer to spend some or all of what you made on a bike for your son, if you think that's appropriate, and stipulate some parameters. Not knowing your son's experience level, I don't know what would be appropriate for him. He likely thinks the bike is cool, fast and that he'll look cool on it. Does he have gear? Real ATGATT gear? If not, I wouldn't even consider any bike for him w/o an agreement that he wear armored gear and helmet when ever he rode the bike. And an agreement with the Ex that she has to be ok with the deal too, and if he breaks the deal, he loses the bike. Not just temporary, but it gets sold the very first time he's caught not wearing full gear when riding it. Don't put the bike in his name either. Or if you do, you or the Ex is the Lien holder until he moves out on his own and pays his own bills. (And don't tell him that so he doesn't think he gets a bike if he moves out.)
Alternatively, if any moto shops are open in your area, see what they will give you on trade and what they have in that price range that might be a acceptable choice for your son. Does he ride dirt? Does that interest him? Is that an activity that he could do where he lives w/o need of transport? If these apply, maybe a dirt bike is a better fit.