So my snake seems to have developed a neurological problem over the past few days. Sunday night I went to pick him up in order to clean his cage, which he responded to by squirming a bit (his usual reaction when I do that) but then suddenly degraded into a spiraling motion with his mouth open. I gently set him back down, where he continued to convulse and twitch for a moment. He righted himself not long after, and began acting normally again. Tongue flicking and so on, as he had before I'd moved him. I was able to clean out the cage later that night. Yesterday, he had two more seizures in the afternoon and evening, hours apart. I was alerted to these by hearing him thunk against the side of the enclosure. Each lasted several seconds. I'd been keeping an eye on him intermittently throughout the day, during which he appeared to be acting relatively normal- moving around the enclosure (casually), flicking his tongue every so often when I'd check in. This morning I found him laying belly-up. Initially I thought he had died, but I went to pick him up and he turned over, still alive and breathing, but lethargic. He's moved and slithered around my hands, but slowly and somewhat erratically (I'm wondering if he's having trouble consistently holding himself upright.) He isn't flicking his tongue. I keep putting his face near the water dish to encourage him to drink, though I haven't seen him do so today.

I got him from a friend in mid-late December, and have had him about a month and a half. He's an Eastern garter. I know that my friend kept him for a couple months before giving him to me, where he was fed pet store feeder minnows. All I know prior to that is that she got him from someone she knows, who got him from "some guy." I don't know his age, whether he was WC, or anything about his history.
He has never, to my knowledge, been fed tilapia or human-grade packaged fish. In my care he's been eating SF Bay brand silversides, and I had planned to start including earthworms and fresh fish fillet as well (though haven't yet.) He's been active, curious, eating like a champ since I got him, defecating and producing urates regularly, and had a perfect shed for me about two weeks ago. He's currently living alone. I did try to feed him last night, as it'd been 5-6 days since his last meal, and he hasn't touched it. Nothing's seemed off about him until two days ago.
He's currently on paper towel bedding (was on aspen when my friend had him) with cardboard hides and a glass water dish.
Ambient temperature ranges from 72-80 degrees, slightly cooler at night.

I've read through a bunch of seizure threads on here, and I don't know what could be causing this. A parasite or thiaminase problem is the only thing I can think of. I've never given him a vitamin supplement. I've only cleaned the enclosure with Dawn dish soap and water, making sure it's fully rinsed and dried before placing him back in it. He's eaten from the same batch/package of silversides the whole time. Nothing has changed in his environment that I'm aware of. Would a vet visit help anything? Would parasite treatment help, or is it too late if that's the case? Is there anything I can do?