Presumably, the parents (H and W) hold the legal title jointly for themselves as joint beneficial tenants.
They could transfer the legal title to themselves and son (S) with H, W and S then holding the title for H, W and S as beneficial tenants in common 25, 25 50%. Transfer of the registered title at LR requiring TR1 completion.
Alternatively, H and W could simply execute a declaration of trust under which they hold the legal title for H, W and S as beneficial tenants in common 25,25 50% with legal title remaining only with H and W. No filing TR1 then necessary.
Under either approach the beneficial joint tenancy of H and W will be severed by so-called “mutual conduct”.
Regarding Paul’s comment about the possibility of the son ceasing to live in the property, if such were to happen the reservation of benefit rules would kick in as the s102B(4) conditions would no longer be satisfied (donors may then need to pay market rent). Not only that, but on any future sale of the property the capital gain attributable to the son would be chargeable to CGT as private residence relief would only apply to the period of occupation (assuming such occupation was qua a sole or main residence).
Whether the gift is a good idea for tax purposes may need careful thought.