Could you try putting him down for his nap somewhere else, farther away from your son so he won't be disturbed. Maybe you could putting a white noise or sound machine(or the radio set in between stations, like Playfelts idea, I think it is) in with your son so he can't hear the other boy. I agree with everyone else that you need to give it a couple of weeks to let these little ones get used to your home and routine. Good Luck!