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Thanks for your enquiry.
You aren't doing anything illegal, so a Solicitor would be happy to act for you to get the paperwork sorted out. Please note however, that it would be normal for your Son to have his own Solicitor and your Mother and yourself would need to appoint another Solicitor, as it would be unprofessional for the same Solicitor to act for all parties due to the nature of the transaction.
Unless you are familiar with Land Registry practice, it will be hard for you to do it yourselves- there is a set Transfer document you would need to prepare and I am guessing your Son will want to protect his financial interest in the property for the remaining monies you will be paying him in the future- normally, this would be done by way of a simple private Charge over the property, which his Solicitor can prepare.
I hope this assists and sets out the legal position. Please let me know if you require any further clarification.