To clarify your questions, since you are considered the landlord and your son the tenant, you will need to file a Schedule E for rental income. You will include the monthly payments received from your son, as rental income. You can then claim expenses against that income, such as interest, taxes, and mortgage insurance premiums paid. You might be able to claim other expenses as they relate to the portion that's being rented, such as (Utilities if they are being included as part of the rent, cleaning, and maintenance, water bill, repairs, etc.).
For better clarification of each question:
1) You won't claim the interest as income. Instead, you will claim the full monthly payment received as rental income. Your son can not deduct mortgage expenses on his income tax. He might be able to get a credit for rent paid if his state allows from rent deduction credit (not all states have this credit)
2) You cannot deduct the principal portion as an expense. You can deduct taxes, insurance premiums, interest, and points paid.
3) Your son cannot claim the taxes since he isn't the owner of the property, nor is he listed on the 1098 mortgage statement.
