@Jen249 As J.Renato says, tell the truth. You can also tell him that while you just started listing on Airbnb, you've been hosting through another site for awhile and do have reviews on there. That you can't post the link to that in a message until his booking is comfirmed, as Airbnb will block it, but that if he books, you'll be happy to email him or text him the link (you will see a coded email address for the guest after a booking is confirmed, as well as his phone number) and if he's not satisfied, he will have 48 hours to cancel the booking with full refund (unless it's a long-term booking).
BTW, does the guest have any reviews? If not, you could point that out- that as you'd be taking a chance on a guest with no reviews, you'd appreciate the same. If he does have reviews, you could point out that at some point when he was new to Airbnb, some host took a chance on him, even though he had no reviews yet.