Dissolving a joint tenancy

Tenancy Types and Management

One of my properties is let to two cousins. They have now gone their separate ways. One cousin wants to take over the tenancy and will have someone to share the house with him so that he can afford it but would prefer to on his own on the tenancy agreement.
Questions/queries include:
Will I have to give him a completely new agreement?
Do I have to include his friend on it or can it just be in his name?
Currently we do not use a deposit scheme as he has been a tenant for 2 yrs, will we have to if we give him a new agreement?
Would appreciate any advice you can give, particularly on the above questions but also on anything related to this issue that I may not have thought of!!