Did he use public money or his own money?
I'm pretty sure it must have been his own private cash, as you can bet that the Telegraph would be baying from the rooftops if they even suspected that it was public money.
If he paid them out of his own pocket, then I don't see a problem here at all.
He's welcome to waste his own money discovering what other people think about him - though the fact that he thought that knowing was worth £5000 pretty much proves that he is a bit of an idiot.
(And the price indicates that the PIs were asking a very small number of people)
Either way, he is right that this has nothing whatsoever to do with the ICO - only the Parliamentary Expenses body (if he did actually use public money) and/or the Police should have anything to say about this.
If the private dicks used unlawful methods, then that's a matter for the police, and if they used lawful methods then there's no case to answer.
Still makes the guy an idiot, but there's no actual *law* against being stupid.