There is nothing specifically to report to distinguish the loan from a gift.
However, if it is really a loan, you must charge interest, and report the interest as taxable income on your return. If you do not charge interest, you must still report as income on your return, the interest you could have charged, using the minimum applicable federal interest rate. (AFR). This is called imputed interest. The current AFR is here
https://www.irs.gov/applicable-federal-rates
I believe that technically, what is assumed to be happening is that you charge interest, report it as income, and then pay it back to the borrower as a new gift. For example, the AFR for December using monthly compounding is 4.21%, that would be $258. You report $258 as taxable income, then you gift the $258 back to your son. Since the give-back of the interest ($3096 per year, or less if he makes payments) is less than the $18,000 threshold, it does not need to be reported.
