how do I search for a reviewers ad to help verify him as a client per se

One, if the client has any posts up, (And you can use the search feature to look for them.) you just click on "see my reviews" which will be next to his handle.

If no reviews, there is a menu item to search reviewers on one of the headers, but I think you have to be VIP to use that.

Do note however, that it is possible for anyone to claim to be someone on TER.  The only way to tell for sure is to have that person PM you with their handle and verify their identity.  You both have to have VIP for that to happen.

Go to Reviews. click on Search reviews and search by his handle.  When his list comes up, you can give him a Whitelist referral.