Sorry for your loss ... I think most of us on here have been in the same place at one time or another. It is never easy loosing one.
It is very good that you are having your other chin checked out to make sure it is OK.
From what you are saying, it sounds like he/she may have had a heart attack due to the seizure, please let us know what the vet says. If this is the case, there was nothing you could have done, even if you had been right there with him when it started.
Our thoughts are with you.
Please notify the breeder you got him from ... the breeder needs to know so he/she can keep an eye on any other sibblings born to that pair to see if they need to take the parents out of breeding.
Jo Ann