I think we are all on your side, we don't want to see you continue to get hurt. It feels challenging because people are suggesting the problem is much easier to solve (by asking him to adapt the vehicle, drive more considerately or to take an additional vehicle, or stay at home) than you feel it is within the dynamics of your relationship.
It is a really concerning dynamic. Where did you learn that you shouldn't disappoint or inconvenience your partner incase he ends the relationship even if you are literally in pain and at risk of further injury?
It feels as if your self esteem is so low that you are scared he will end the relationship over you putting your health and comfort above your involvement in his hobby OR he is so controlling and scary or guilt inducing that you cannot raise this with him OR some combination of the two issues.
The bit about being flung around in the back seat with your son (how old, is he safely restrained?) It sounds like you are scared of him. Because the rest of us are thinking we'd tell him to slow down and never step foot in the vehicle again if he refused.
Trying to get to the bottom of whether he is callously uncaring about your health and safety or whether you just aren't confident enough to assert your needs.
Either way, talking things through with a counsellor or sensible empathetic friend might help.
If he is uncaring then you and your son are better off with the relationship ended. If the latter then you need help and support to realise that his love for you is not that fragile or fickle.
I had depression, low self esteem and high anxiety when I was younger and it meant I was passive and put my idea of what my husband wanted above my own. I worried about disappointing him if I wasn't as enthusiastic about his interests. I have become more confident over the years and realised that it is absolutely ok to have my own hobbies, opinions and needs. And I am much better at asserting them these days but probably still have a way to go.