Probably nothing. As I said, I've never been asked to show it.
At risk of sticking my nose in where it doesn't belong, however....
If you are trying to build a working relationship with your ex re: taking your son to live in the UK with you, it would be to your advantage to try and work with him on this two week visit. Don't give your ex any ammo to use against you.
As I said, really none of my business, but I am negotiating w/my ex right now over taking our kids to live in Scotland and as I also said, he can be a real b****rd at times. I know how frustrating it can be. But it's really important to be up front with everything. It's not even so much the idea of getting 'permission' (I HATE that term!) as it is that he be fully informed about your plans. That way he can't come back later and say "I can't trust her, she keeps trying to run with our son, took him out of the country without informing me, blah, blah, blah....."
Just my totally unsolicited 2p...
(Hee hee, I love PITA too....I have a few more acronyms for him but I probably shouldn't share them here!!!)