When you said he goes rigid, does he actually fall in his side? Does he jerk really bad? Does his whole body twitch or just his head? If he isn't doing these things, I don't think it seizures, especially since he only does it when you take him out of the cage on occasion. If it was seizures it would happen more frequently, and definitely inside and outside the cage. When gliders are scared or very stressed, they can go into almost like a trance state. They will become stiff, not move from their position, stare off space like they are in a trance, and their head can sometimes twitch a bit or even their tail. Once you get him back in a pouch how does he act? How long does it take him to come out of this? Is it right away, or can take a could seconds to minutes? What does he do? Also, could you explain a little more about the frothing at the mouth?
Also, when this happens, what happens right before he acts this way? A loud noise,a sudden movement? Do you have other animals in the home? Are they close by at those moments to where he all of sudden smells them, or do you have a unfamiliar scent he is Smelling on you? Try to pay attention to what is happening in his space that could be causing him to become very scared and stressed out.
You said something about diet. This could also be very possible to cause this to happen. What OHPW Recipe are you following? And what else are you feeding him? Do you add extra vitamins? Or substitute certain things in the recipe sometimes? What treats do you feed them?
And one more question, what do you keep in there cage? What kind of bedding do you use? Do you have wood in their cage? What do you line their cage with? And what do you clean their cage and bedding with?
Everything I asked you is so very important I promise. I'm not asking these things just because. I'm asking because all these things could play a role in what you are seeing with your glider. I really don't think your glider is having seizures, so I would like to help you try and pinpoint the source to this behavior. It could be a very very mild seizure, but if it is, I don't think it is neurological. I think there is something causing it. And if I can pinpoint what it is, you could get them to stop completely by just changing 1 simple thing you would have never had thought would be the reason for this behavior.
And I'm sorry, I know I'm writing you a book but I'm honestly just trying to be helpful and figure out what is really going on with your little guy. I'm not a vet, but a lot vets, even exotic ones don't know all the Ins and outs when it comes to gliders because they don't specialize in them. A vet can do all the testing, surgeries and other things like that. But as far as diet, and things like that, they don't really know the true facts because they don't work with them everyday, only once in a blue moon will they come across a glider. They look stuff up on the internet to answer your questions when it comes to things like that, and everything you research on gliders is so contradicting its hard to get the facts. That's why I got a mentor from a Facebook group injoined that has had the animals for 20+ years, and knows the Definite dos and don't. So I'm honestly just trying to help you. I have 9 gliders, and I've rescued more then that. I've never had a sick glider. I'm also an approved vendor for suger gliders, meaning I sew and sell safe an approved cage sets, and glider toys.