Cerise rides in the Snoozer Lookout car seat. Lately she does whine sometimes to be up front with me and hangs her head over the front of her seat trying to get my sympathy

but I tell her "No, sit" and she stops.
I would put him in the car seat and repeat "No, sit" a hundred times because it is best for him and eventually he'll realize that he has to sit and enjoy the ride. You might place a Bully Stick and a favorite squeaky toy in the seat with him