If it was me, I'd split it into two payments, one for rent and a separate one for shared bills and formalise the rental agreement. Be very careful in terms of his contribution towards your shared home as opposed to his rental agreement with you; so in other words money, labour or time that he puts towards furnishing, decorating or maintaining is treating the place as his property - perhaps for the moment don't buy things together as this muddies the water considerably.
One thing to bear in mind is the tax implications of a rental agreement, I am guessing that you can avail of the rent-a-room scheme in this regard.
One thing to bear in mind is the tax implications of a rental agreement, I am guessing that you can avail of the rent-a-room scheme in this regard.