Consult a lawyer!
I've been in a similar position as you, in VA. You may find that he doesn't have legal ownership of either gun anyway, and I certainly doubt that he'll take you to small claims court over two guns that he can't own (cheaper just to get mommy to straw purchase him another gun, not condoning that mind you, but you said she'll do whatever he says). You will probably lose all hope of seeing your grandkids again while they're young, but at least you wouldn't have contributed to a dangerous situation for them.
Perhaps your son only wants to sell the guns and get the cash anyway. So, tell him/them you plan to sell them right away and give him the money. See if he's ok with that. You may be able to save your relationship with your grandkids that way.
I'm in Stafford too. PM me if you want to talk!
Liberty and freedom often offends those who understand neither.