You do not need to issue a 1098 to your son since you are not in the business of making loans.  If you would like to issue a 1098 you can, but you are not required.

You report the loan interest under Federal Taxes - Wages & Income - Interest and Dividends.  If you do not see the Interest and Dividends section please select the down arrow beside "Help me add other income" and then "Jump to full list".

The interest on the loan is entered in the same area as interest reported on a 1099-Int.  The TurboTax entry screens assume you have a 1099-Int but you can enter interest here without a 1099.  On the first screen say "Yes" and then on the next screen say "I'll type it in myself".  The entry screen is set up to resemble a 1099-Int but this is where you will enter the income.  In the Received from box enter your son's name.  In the Interest income box enter your interest income amount.