My name is XXXXX XXXXX I am one of the UK tax experts here on Just Answer
I hope you do not mind me interjecting, but as you have a relationship with this individual, there is no rental income to declare - he is merely paying you towards his share of the costs to live there. (even if the amount you have arrived at for a payable purposes, was undertaken using a fair rental charge calculation)
So, as there are no rents to declare, you need not worry about the associated expenses one usually has to consider, when in receipt of rental income.
Please do feel free to ask any follow up questions.