Can you tell me what you would like to know please.
It would be useful if I knew what the question is before I call.
Your son will have to pay the stamp duty surcharge as you refer to it. See Example 9 here which mirrors his situation pretty much. This contrasts with Example 10 where the owner has 10 buy to lets and the property he lives in and because he is selling the property he lives in and buying a new home to live in, he won't have to pay the surcharge. That is the inherent unfairness.
I hope this helps but let me know if you have any further questions.