@NCperson
"Sharing" is likely the wrong term as this is essentially a below-market-rate situation (i.e., the son is using the property as his principal residence but not paying full FMV).
No matter how you view this, it is NOT a scenario where reporting the mortgage payments as income, considering the use of the property as part gift, or whatever is going to make a difference on the OP or the family member's tax returns.
