Here is the difference:
If you and the room mate were paying rent to the landlord directly together, then you do not have to report income.
However since you pay rent to the landlord on your lease with him, and then rent space to a room mate, then you have to report that as income.
Likewise, if it is your lease and rent obligation for which you alone are obligated, and the room mate pays the landlord for you, then you still have to report it as income.
In order to avoid this as income, you and the room mate have to be legally obligated for the basic rent and pay your fair shares to the landlord.
Under your scenario you would be required to report the rent. However you are also entitled to take certain expenses.
If your room mate also pays part of your utilities, that is also considered income.
So here is how it would work.
1. Report income received from room mate on schedule E.
2. Then take the following expenses as a minimum:
a. For the bathroom and master bedroom that he uses by himself: apportion the floor space by percentage of the whole apartment. I.e. Apartment is 800 square feet, and the bath room and master bedroom are 300 square feet, Then 300/800 square feet X the amount of rent you pay for the entire apartment is expense.
b. Apportion the utilities between you and him (common area).
Because there are no separate entrances, and the property is also personal use, you can not take more than the income as expenses. (no passive loss carryovers as it would be with a regular rental unit)
Please see this publication, in particular section called personal use property or dual use property: http://www.irs.gov/pub/irs-pdf/p527.pdf