There are quite a few things which can cause seizure like symptoms so unless you put him in the fridge for a PM you will probably never be sure.
At 3 years old it seems unlikely to have been anything physiological wrong with him from birth. The most common triggers that seem to come out of nowhere are chemical poisoning or overheating.
But there are also parasites that can cause neuro problems ( some ameobas and protozoans iirc ).
Did you recently add any new snakes to your collection?
Hmm interesting, I mean I temperature & humidity was good as usual. Chemical wise I don't know, I haven't used any chemicals around it or anything. & as for it being a parasite, how would it have gotten it?
& no I have not added any new snakes, he was the only one I had