Deductions & credits

Thank you but what you suggested does not seem to work because on Turbo Tax Online (which I am using), because when you click on "None of these Apply" it takes you to a screen to fill in the mortgage detail info that says to fill in the info from your Form 1098 and then it puts the mortgage interest expense on Line 8a on Schedule A which is the line for when you have a Form 1098. As you know, there is no form 1098 in this case. Here is the screen where I can click on "None of these Apply" as you suggested:

Do any uncommon situations apply?

Someone co-owns this loan, and I only pay a portion of it.
The seller is financing this loan and I didn't receive a 1098.
None of these apply.
Now here is the next screen after clicking on "None of these Apply":
"Let's get the details from your 1098" "Box 1 - Mortgage interest"
So how does Turbo Tax want me to enter this mortgage interest expense for which no form 1098 exists and the lender is not the seller (i.e., my situation)? As an aside, we know that the amount of my son's mortgage interest expense deduction has to equal my interest income reported from him on my return. The only issue in question is how to enter his mortgage interest expense.