Yes, that is correct. The IRS goes by the character of the payments, not on what you call it. Call it what you want, the payment is in exchange for rent of space or use of space on the premises. In exchange of space that you are legally responsible for by virtue of the lease you hold with the landlord.
A rose is a rose by any other name, right? Here is an example unrelated. Sometimes it is easier to see it at work in an unrelated situation. Regarding character of use or payment.
The IRS has rules that: If a person skins his car to advertise the business, and that car is used for both personal and business reasons, the skin does not convert the car to a business expense. It is the character of the car being used to go to the grocery store as a personal use, and to drive clients around as a business use. The so called 24 and 7 advertising on the car does not change the character of how the car is used. The expenses have to be apportioned.
The character of your situation is that he pays you money and half of the utilities, in exchange for the private use of one room, and shared use of a common area. You can call it what you want, but the IRS calls it rent.
Yes, because you use the common area together, you are using the common area, and you can only expenses the area he sleeps in. BUT there are some minor expenses you may be able to apportion 50%. Here is an example: If you hired a maid to clean the apartment. You can apportion the cost of the maid to the percentage of his room plus 50% of the common area. You could apportion the utilities to the same square footage apportionment.
It would be no more work whether you had a simple sublet or not. You have a sublet agreement right now, either written or verbal. It does not matter. As long as he is paying you for service, whether you call it sublet or call it a room mate would not matter. You have the same exact situation.
What would be easier is having him added to the lease you have with the landlord, and make his portion of the co tenant lease be 800 dollars. AND then for you to have the utility bill put into your and his name. then you are both legally obligated for the rent and you are both legally obligated to the utilities to the same person (landlord and utility company).
I can not tell you in this environment that many people in your situation do not bother with this. I would like to but I cann't.
NOTE: informatin provided is not intended for use of avoiding tax obligations.