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This is a Potentially Exempt Transfer (PET) and there will be no Inheritance Tax (IHT) liability so far as your son lived 7 years plus after gifting you the money.
If he dies before 7 years after the gift then the tax will be tapered depending on how long he lived as per below.
Years between gift and death Tax paid
less than 3 40%
3 to 4 32%
4 to 5 24%
5 to 6 16%
6 to 7 8%
7 or more 0%
Hope that answers your question but please let me know should you have further questions.