I'd definitely get him tested them. I overnighted Vega's hair samples and I got an e-mail and then a letter with her results. It took less than a week to get her results back.
I don't mean to butt heads with you, but on the AQHA, they have it has HYPP, and I think they'd have it the right way, and I've found more information on it by typing in hypp than hpp.
I've posted this link a lot, and I found it extremely helpful. http://www.bringinglighttohypp.org/
Also giving him Karo Syrup is said to help if he's having an attack, but maybe try giving some to him and see if it helps out or not.
I'd also try and get him a new diet ASAP too.