@Alison4:
I would call Airbnb and explain the situation just as you did here. I agree with you, I wouldn't let him know the truth. Sometimes, some things are better left unsaid.
Had a situation where we weren't comfortable hosting a guest. When I selected not comfortable hosting guest, there was a blank box with something like explain or why? I thought my response would be going directly to Airbnb but it ended up sending as a message from me to the guest. I had entered:
"Aside from not having any reviews, we mainly don't feel comfortable as he has not gone through the verification process, doesn't have a profile picture of profile description."
Fortunately, the guest understood and ended up adding a line on his profile description and a picture. We ended up receiving another request to book and did not end up hosting him.
You can also just decline and not give an explanation OR block one of the dates from his reservation inquiry so it will automatically turn into a not possible.
Let us know what you decide and how it goes.