"How is this handled from a tax situation?"
Simple answer, you ignore it. You have a second home, that your son happens to live in and share expenses. You may be able to deduct the mortgage interest on your personal tax return (you can deduct mortgage interest on your "main home" and one second home, up to a limit of $750,000 of mortgage balance). You may be able to deduct the property taxes on your personal tax return, up to the $10,000 cap on all state and local property and income taxes. You ignore expenses, depreciation, and anything else.
To treat the home as a rental on schedule E, which would allow you to deduct your expenses including mortgage, insurance, utilities and depreciation, you would have to rent for profit, at the fair market rent (what you or another landlord would charge an unrelated person for the same or similar property in the same area). You said you aren't doing that, and there are some good reasons for not doing it that way when renting to family.
So best to treat it as family living in your other home and helping with expenses.
