Very simple.
Your son signs up with Sipgate.co.uk, which will give him VoIP, and most importantly, a UK number in the form (outside London and the big cities) 01xxx xxxxxx. He needs a £10 mic/headset which he plugs into his laptop. You agree a time for a call, and you dial his number, which for all the world looks like a regular phone number. He answers it.
You probably get free evenings and weekend calls from your phone co., so it costs you nothing to call.
If you like to try it out, PM me, and I'll give you my full Sipgate number, which starts 01491 81xxxx.
If he can't answer, you can leave a voicemail, which gets forwarded to him in an email with an attachment he can listen to.