I'm assuming no money is changing hands and you have a freehold property and no lease is involved and that the property is registered at the Land Registry and has a title number? If not let me know a I'll re-advise.
Do you have a mortgage on the property, if so the lender would have to agree as you're giving someone else a legal interest in the property which forms their security for the money borrowed.
The Land Registry will more than likely need the mortgage lender's consent in order to register the transfer and change the title. Usually there is a restriction placed on the title register by the lender stating that no disposition (eg sale or change of ownership or further charge) cn be registered without the lender's written consent.
A property transfer is quite straightforward and a very quick job, a TR1 form which is from say your name to the name of you and your son. All parties will need to agree and sign for the transfer to be effective (unless trust rules apply but I'm guessing this is unlikely in this case).
All parties who currently own the property should have the choice of independant legal advice about decreasing their share of ownership or interest in the property by adding another owner. The solicitor should deal with this.
It shouldn't cost much at all, I've done them in the past for about �100-�150.00 plus VAT.
The solicitor would request the consent from your lender, draw up the form, arrange for it to be signed by all relevant parties, deal with any stamp duty forms if required and sent the form to the Land Registry.
There will be a Land Registry registration fee of �40.00 payable.
Example TR1 -
http://www.landregistry.gov.uk/assets/library/ documents/tr1.pdf
Example AP1 -
18:29 Fri 23rd Feb 2007