@M-MTax while I appreciate you are done and won't respond, I did take the time to read the link you provided.
The personal use is only how to apportion the expenses. If there are personal use days, the expenses can not be deducted. If there are rental days, the expenses can be deducted. I see no discussion in this section about reporting or not reporting the rental income. Since the son is paying less than market value, all the days the dwelling is used as a home are "personal use days". That means none of the expenses (other than mortgage and property tax are deductible and those expenses can only be deducted on Sch A).
The rental income discussion is further up in the link (and I already posted that).
then further in the link there is this discussion:
Reporting Income and Deductions
Used as a home and rented 15 days or more.
If you use a dwelling unit as a home and rent it 15 days or more during the year, include all your rental income in your income. Because you used the dwelling unit for personal purposes, you must divide your expenses between the rental use and the personal use, as described earlier in this chapter under Dividing Expenses. The expenses for personal use aren’t deductible as rental expenses.
