How should this be shown in the accounts and tax returns?
Disallow the expenditure (interest, repairs, utilities etc) on this property in the same way that you would deal with the private element of motoring expenses.
I really have no idea if it is to be taken as benefit in kind on his tax return
BIKs are for employees. In this situation disallowing the expenditure will bring about the correct outcome.
so therefore what should happen on my tax return as effectively I am 'losing' my half of the rental income on this house, can I claim it back?
There is nothing to "claim back". This is an arrangement between the two of you, and there is no adjustment possible in respect of rent which you will not be receiving as a result.