1) You can only charge him what the terms of the mortgage/agreement allow you to charge, so if the agreement says you can only charge late fees on the interest-only monthly payments, you can't charge him a late fee on the principal. Review the agreement to see what you can and can't charge--again, you can only charge things which the agreement expressly says you can.2) You can't charge late fees on late fees.3) The statute of limitations (or time within which to sue) in your state for written agreements is 5 years; you can potentially sue for anything owed & unpaid, including late fees, from up to 5 years ago from when you file the suit.