RED FLAG. This is either:
(1) negotiation tactic if he does not trust you or he thinks "market rate" is lower
(2) he needs to show management because they don't believe him.
(3) indication that you are asking for more than budgeted (you're the odd guy) and maybe he needs it to prove this to his management to get you on board.
(4) he's fishing for information because he really does not know the market rate (worst case because he may or may not really want you)
As rude as it is... clients will ask. But also try to work it out with him because its unlikely that he's TRYING to be a jerk (at least I hope not for your sake!).
a) Ask him why he would like this? You need to know what's in his head.
b) Go ahead and tell him that this is an unusual request. Because it is!
c) If you feel uncomfortable about it - don't disclose it. If he really needs you - he'll pay what you ask him to pay.
Other things to consider:
Does your old NDA govern your disclosure: If you have an NDA with the past client, then its fair to say that you have a non-disclosure that governs your client's privacy including contracts and other documents like invoices. Therefore - you are not in a position to release this information because it shares what they pay for services.
Explain that the rate is your current expectation based on market rate and your skills. Discussing it with your past clients is irrelevant. It's like going to the gas station and asking to pay last months prices. (Ha)
Ask if the disclosure of this confidential information required to get the job? If it is, and you're uncomfortable with it, then you should think twice about joining.
Last - tell him that you're an honest guy and your reputation is important. If he does not trust you, give him the opportunity to talk with your references.