I don't think that it has anything to do with his behavior, because even kids without a fractured skull can still smart mouth their mother. Even what seems to be the sweetest child in the world will have an attitude once in awhile. If you want to gain more respect from your child, I suggest you see a professional and ask for advice that can help discipline your son.
Still, I don't think the fractured skull has anything to do with this.
I dont know if the injury can have long lasting effects on his behavior but first if you are concerned about that get a further evaluation of him. What about the Parent/child interaction here how do you cope with the bad behavior, what is the discipline guideline you adhere to,seems you are asking about a Cats Scan more than resolving any parenting issues.