I'd definitely have your MD chk it & maybe order an mri
In 2010 I left the bar, simply stretched & popped my neck walking to the car, causing me to break my neck. C5 & C6 w/ herniated disc between.
it took several minutes to stand up straight getting out of bed. Went to work for a week. Then I saw my MD. Who ordered an emergency MRI. Excruciating pain in neck , back & left hand. Yes, I have a very high pain tolerance.

I'm not saying he has a herniated disc in his back, or bone break, but best to check

The cause was severe osteoporosis caused by 50 yrs of Phenytoin (Dilantin)

I'll be praying for the best

I can only suggest if he still has back issues to have back looked at by regular MD as they can give a better idea what's going on. This might not be a desired visit but I use to damage myself a lot when having seizures so I can recall the many doctor visits that were made. I still have the marks on my fingers and legs to remind me I've done the damage. 🙃
Anyway, I do hope things get straightened out and they get the feeling better soon! 💓

Pain and body aches are common after a seizure. It is from the muscles clenching up.

I've also broken T6 which caused severe back pain, by lifting weights at the gym, but you get the picture

I wish him well, I kinda had that happen to me this morning, I do have nocturnal ones time to time, but my thing is, sometimes I wonder if I did, I have a upper back discomfort this morning and I don't know what happened, I'll be ok but sometimes I know it happens and sometimes not, my opinion