Is there any chance you could work with your son and your current dogs to make one of them HIS dog?Do not see where you posted the age of your son.8 MO GSD requires kinda some maturity-I would guess 8 YO at least.(9 YO border collie might with some effort from you work out regardless of son's age)Whatever son's age the best lesson you could teach him(forgive 60's reference)is 'Love the one your with'