I charge 765 monthly this covers the mortgage and taxes. What about insurance and sewer taxs I rent to my son

Assuming you are talking about a separate house or apartment:
None of the above, except insurance, which has to be fire and liability, not HO.
You declare the income.
You deduct expenses for repairs.
You don't deduct improvements.
Mortgage interest goes under interest deduction.
You depreciate the place as an investment property. This is a double edged sword, because then when you sell, you owe a whopping capital gains tax. Unless you get around that by living in it for some number of years before you sell it. That number is currently.. I forget, 3 out of the last 5? Another way around it is to NOT accept rent from him for a minimum of 30 days a year. Technically someone else should move in then, and this could get you in hot water. Pick up a tax guide at a PO, read it, and then consult an advisor if you have an unusual arrangement and want the best advice.

If he is just renting a room in your house, you assign all the expenses a fraction based on square footage.

The fact that it's your son who is renting your property doesn't matter - you are essentially in the business of managing a rental property, and you must report your income and expenses for this business on Schedule E (or perhaps Schedule C). Your income from the business is the rent received, and your expenses include the operating costs you mention as well as any repair costs and depreciation you take on the property. You should consider using a tax pro, or do some digging into how one handles rental properties. You can start here: Tax Topics - Topic 414 Rental Income and Expenses (http://www.irs.gov/taxtopics/tc414.html)