It is understandable that he wants to spend as much of his weekend as possible with his children. Regardless, your ex-husband may have a desire to somehow include himself in this situation. If so, it would be helpful for him to express this in a cooperative manner that revolved around his son's needs. I am not saying that his desire to father his son through an illness could not benefit his son. What I am saying, is that your children are required to make significant adjustments when sharing time with their father.
Seizures are traumatic medical events.
In fact, I find myself wondering why he didn't thank you for taking such good care of his son and for bringing him over so soon after he awoke. In California, for example, it is possible to share legal custody with your ex-spouse, but be granted full physical custody for your child. Joint legal custody secures parental rights, while full physical custody may be granted to a parent with whom the child resides most of the time. However in California, if you have full physical custody of your child, your decisions generally stand in the case of conflict. Consult an attorney for advice regarding the interpretation of your custody arrangement in the case of your son's seizures.