You send him a letter stating he is in breach and if he does not stop you will sue.
It would be best if it comes from a Solicitor that practices in Intellectual Property.
You can search the LawSociety website
You should also work out what you are losing by his actions and this should also be in the letter.
If you choose to take action the following link sets out what you have to prove
This one is the process
But you have to send them a letter to ask them to stop as a start
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