My first observation would be that staying in his car can't be fun! Who would really prefer to sit in a car in a driveway rather than enjoy their home. My H escapes too, but he does this in bits and spurts by escaping to his studio to play guitar or by going to the gym. The effect is the same though. Even now that he has started therapy, it hasn't changed his need to be alone, nor did I expect it to. This time is a relief from the mask he has to put on for the world. Is it fair that he gets more time than me, no... But it isn't about fairness, it's about what we need. He needs a safe place to escape (not a car) and you need more help. Set down boundaries like "I need a break from 8-9pm every night" where you take over. Insist of him getting therapy WITH CONSEQUENCES YOU WILL KEEP, even if they are soft. This will let him know you mean what you say. But realize that it will be a long time before he will be the husband that comes home, engages you and your 3 month old and wants to here all about your day. I know what it was like to have babies, and I don't mean to sound like you shouldn't ask for what you need, but if you don't or won't leave, then you have two choices: work within what he is capable of on any given day or get outside help.
I know its hard to be a de facto single mom... I've been there. But if his escape is a car, my guess is that he is as unsatisfied with this situation as you are, he is just afraid to take the risks to fix it. He probably feels like taking on recovery will completely crush him and he will become someone you have to take care of along with your son. He might feel this is the lesser Of two evils because he is at least able to function enough to work. He needs to feel that if he takes this risk to start recovery and risk falling apart for a while that you will be able to stand beside him.
Good luck and keep talking to him. Try not to let him engage you in argument and find a healthy outlet for your resentment... but keep talking and reminding him how dysfunctional he has become.
I am not your rolling wheels, I am the highway
I am not your carpet ride, I am the sky