This sounds like a very strange story all round.
Firstly, did your son open the car door while the car was moving? If so do you know why, what the circumstances were etc? Because that sounds extremely dangerous.
I think it was pretty normal, even appropriate, for his dad to become angry and even shout at that point. Any parent would, surely?
On the basis of this incident, and your son adding that his dad hit him, you insisted the dad move out and gave him limited access to his kids - it seems a very extreme reaction on your part..... I have a lot of questions really.
In what way did he say he had been hit? Hitting a kid, hitting anyone, is never ok of course, but in such an extreme, potentially life endangering event one can see how it might unfortunately happen. Was it a slap at the time, sort of out of panic? Or did he stop the car and then punch him in the face? What?
Did you consider believing your partner at the time, or just automatically believe your son? If the latter, why?
Had you been questioning the relationship before? Or was it fine until this point?
How do you feel he should have reacted to your son opening the car door while he was driving?
I am just wondering if everything is ok with you, OP? Because it seems such a strong reaction.
Very concerning to hear all that has happened to your partner since. I very much hope he is OK. Are there any other ways you can reach out to him - any other friends or family you can contact? Any places you know he loves you could look for him? Any social media he might still be checking?
Can the police check his bank and see if he has still been withdrawing money?
At the end of the day, if the worst has happened and he has harmed himself, it is NOT your fault. Ultimately everybody is responsible for their own actions.
However, for the sake of your son's, it does sound like, providing your partner does come back and is ok, you might BOTH benefit from some form of counselling to work out more positive ways of co-parenting going forwards (be that as a couple of separated), that ensure everybody is listened to and everybody's needs are considered.
I wish you all the best.