B.M.
Hi L.,
I don't know if you've moved him from a carrier car seat to a convertible one, but my youngest HATED his carrier car seat and at about 3 months, we moved him to the convertible car seat and he was much happier.
I don't know what it was about that seat, but he SCREAMED every time he was in it.
I hope you find a solution...I know how it is to dread the car rides!!
B.