S.T.
Is this jump seat sideways or facing backward? If his seat does not face forward then it will not protect him properly from an accident. I was looking into buying one of those old time station wagons with the jump seats in the back/sides and found out that the safety seats absolutely must never be used facing backward or sideways.
They do not recommend children EVER sitting in the front seat, even with no air bags. If PawPaw has another option then I would highly recommend that you tell him that he must ALWAYS ride in such and such truck and install the carseat where you want him to sit. After ONE TIME of my parents improperly transporting my children I made sure to always do the seats myself. Except once= they picked us up from the side of the road where we had been in a terrible car accident. My parents took the kids and I stayed to talk to the sheriff. Well, they didn't do the carseat right and my already traumatized 2 year old fell in the floor!