This may be naive of me, but I believe the interest rate would have to be decided by a written agreement. Otherwise, it would be understood, even by a courts standards, that the loan is expected to be paid back, but not with interest. He would also be admitting that he would be gaining a profit on the loan and would have to claim that on his taxes. And you could claim the interest paid on the loan...but that's a whole other rat hole you may both want to avoid.
If he wants interest then he should have you sign a contractual agreement. Again, I could be wrong. I just deal in cash and then deny, deny, deny.....