Hello and welcome to the site. Thank you for your question.
1. CGT. There has been a disposal in July 2013, but as it is to a related party we can cannot claim to have made a loss on the disposal.
There is no disposal in Jul 2013. All you have done is to convert the loan into a gift by cancelling the loan note and the associated charge.
So, CGT does not come into play at this point.
2. IHT. A PET was made in July 2013 of the value of the loan note, that is £200K
You are correct that this amount £200k is regarded as a potentially exempt transfer for IHT purposes and the seven year rule applies.
3. There is no tax impliaction for my son, who has occupied the house throughout, is the legal owner and it is his PPR.
There are no tax implications for your son by your aforementioned actions.
I hope this is helpful and answers your question.