I drive a truck (with running boards). Liam can, with assistance, climb most of the way into his seat. I do have to help a little, but I dont have to support his full weight. It also doesnt happen very fast cause it's kind of like rock climbing for him. Honestly, I dont see a huge difference getting them in FF/RF. You might try making it kind of a game, put something fun he gets to play with/eat/etc in the seat and then carefully help him get up into his seat. It's just an idea, but with the hyperemesis, the first thing I lose is muscle tone so Liam's had to learn to help out when getting in his seat.
Just a few thoughts since our kiddos are the same age! What kind of vehicle do you drive and where is his seat? Would a different placement in the car help? I've alternated between having Liam in the middle (so I can sit next to him while buckling him in) and moving him outboard to make it easier for him to climb in himself.