DS1 turned forwards at 3 1/2 because we had been on a long journey and our hire seat was forward facing so he was used to it, and we needed that seat for DS2. Otherwise he'd have been rear facing until at least 4. DS 2 is almost 2 and will be RF for at least a couple more years. We have a Maxi-Cosi Mobi.
For long journeys when they were small, I had a basket full of toys (age appropriate and safe to play with unsupervised). Each time they started fussing they got a toy passed over. When they dropped it/threw it away, I waited a bit, then handed over another. We could travel for an hour or two, before a break when I picked up all the stuff into my basket again. Things like a few baby links, Lamaze style soft toys with scrunchy bits etc, a plastic pot with a hinged lid they can open and close, small board book you don't care if they chew, rattle, another few baby links, two pots that fit one inside the other, etc.
Obviously that's not going to help if he's hysterical at the concept of being put into his seat.
I agree with other posters, a new, higher seat, may make a big difference. If you get a new seat, introduce it at home, in a happy environment with you sitting in front and playing with him. Sit him in it with no straps, play, laugh, stop. Another day, sit him in it and do up the straps, play, laugh, sing, do stuff you know makes him giggle. Stop soon and let him out. Do that a few times before you put him in it in the car.
The first few journeys you go on, rig it so they are short, and play with him as soon as you get him out.
Get a good mirror that he can see you in.
His distress is a safety risk, but turning him forwards too soon is also a big risk.
Good luck!