If he wants a non-disclosure signed by you, he should be happy to send you one. I wouldn't jump through hoops for him. Honestly, to me, it just doesn't sound professional.
On the other hand, maybe this is how he is trying to weed out tire kickers. Are you a serious buyer and have the cash to buy it?
I had an issue with a lead I had gotten from a wholesaler that I work with on my last purchase and this is my big problem with wholesalers in general. I got the lead and had some interest. A couple days later another wholesaler said that they had a property in my specific area. It was the same property that the first wholesaler had, but, it appeared to me like he was marketing like he had it under contract. I know that some of these guys work together and help each other out, but I think that stuff should be disclosed (i.e. "I'm helping someone out on this one"). At least once a week, I get emails of the same property from different wholesalers. Anyways, I think this is one of the many reasons wholesalers get a bad reputation.
Good luck.