Thank you for your question.
Your son's share of the gain would be chargeable to CGT as the property has not been his main residence or primary property at any point over the period of ownership, I'm afraid.
More information on who qualifies for relief for CGT purposes can be found here
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.