My children had high fever convulsions. It seems that it runs in the family as my little great grand daughter had one recently.
It is more accurately caused by a fast rising fever. So at the first sign of a fever it is good to give him the Tylenol. If it is high already give put him in a lukewarm bath and letting it cool will keep him more comfortable. Keeping his head cool is helpful too. Warm covers will make the fever higher.
Your son might have been weakened by the seizures so maybe that is why he wanted to be held.
These are usually not harmful and if you keep watch whenever he has a fever it should be ok.
The key is to keep the tylenol every 4 hours around the clock. This way the fever will not spike.
Not your fault..especially if this was not a problem with your other kids.