@res6cueAfter your son lost his phone and you got him a replacement. I assume you had also got a new SIM card too? Have you paired the SIM card to your son's account via selfserve? If not log ino and change the SIM card to the 19 digit of your son's new SIM card #. You need to do that in order for the SIM card to work.
It sounds like your iPhone is already unlocked.
a) Reboot the new phone with the PM SIM installed. What message appears on the phone?
b) Try the PM SIM in another phone?
c) When he lost the phone did he report it lost in his PM account?