Concussion with tremors

My son got a concussion playing football and started shaking(whole body) it remained for approx. 3 hours. Took him to the hospital and ct was clear.  He returned to play a week later, maybe too soon, and after the 4th hit started shaking again(this time only an hour).  Took him again to the hospital and they did a ct of his neck and that was clear.  They seemed to not be concerned with the tremors. I then took him to his doctor the following day and he also was not concerned with the tremors.  He has played for 7 years and never had anything like this before. He is 16 and will be a senior next year.  Is this normal with a concussion. Do we need to be concerned with this.