Ask your son (in-person/phone call) to check the blocked contact list on his phone. If he doesn't know where that option is (if the phone even has it) tell him to refer to the phones user manual or Google how to find it. I mention this because some phones have a 'block contact' option placed conveniently near the reply option.
- - - - - - Any opinions expressed by myself are entirely my own and do not represent Virgin Media in any way.
Changing the sim won't affect any games or apps in the phone. The only thing this may affect is contacts. Can you let me know which phone your son is using so I can show you how to save the contacts to the phone?