Well, I do have a Barrett 50 but I also think it is a bad idea. You're putting the kid in the middle and that's never good. And I did see a young person get their brain scrambled a bit a few years ago when they was sitting near a .50 BMG when fired. That's right, not shooting it but beside it. It's a scary powerful weapon, and the muzzle break blows a lot of the force out sideways.
Having said that... I also have numerous AR's, bolt action .223, numerous .22 rifles, .308 rifle in bolt and semi, plenty of cool/fun pistols and more. I'm happy to have you and your stepson to my club in metrowest as my guests to shoot any and all of these, once he's gained confidence and shows proficiency and interest. I've got plenty of ammo too... and I love to see kids properly introduced to our sport.
Most kids are thrilled shooting a .223. The kick is minimal but the look of an AR gets them all excited.
Offer is on the table.
But do the right thing for the kid, not for revenge against the other guy.