Craig, you would have to create an account in your sons name and do the purchase using his current location information for an FFL. It would be easier to purchase your son a Buds gift card and allow him to select and purchase the pistol. Since you want it registered in his name, he would have to do the paperwork on or after his 21st b-day hits to be the purchaser/ registrant. I say this only because you don't want any red flags implying a "Straw Purchase" .

I would purchase him a gift card in the amount of the gun and have and on his birthday give him the card and let him order it using the gift card

Hi Craig, you would have to create an account in your sons name but with your email address and do the purchase using his current location information for an FFL.