I'm really sorry OP, and I can totally understand you wanting it to be true, but I just really don't think that it is.
It was a very cold night last night. There is no way that he would have driven all the way to the end of your road (whilst drunk and high) and then decided to sleep in a freezing cold car all night, knowing that a warm bed awaited him a few metres away.
How would you know if he'd been taking coke? He could have just sneaked in late and slept on the sofa. Surely even if he had taken coke, he could still appreciate that staying out all night would be even worse.
If he was taking coke, who was he with? And until what time? Because he wasn't doing it on his own. And if he was doing it with friends then surely, if he was genuinely scared of coming home, he would have stayed at a friend's house? And he could have recharged his phone and sent you a message telling you that he was drunk and staying at a friends.
There are just so many ways that he could have concealed his drug use if he had wanted. Or he could have just told you that he was totally drunk and fell asleep somewhere.
So, my question is...why, if he was too scared to come home in a bit of a state, was he not too scared to admit to taking coke? And not too scared to admit driving under the influence? It makes no sense. Aside from the more likely he is using it as a diversionary tactic.
I think that you're getting half the story here.
Put your cards on the table. Explain that you are not convinced and tell him your fears and then get him to prove to you. He needs to produce evidence to back up his story!
Phone records, receipts for clubs/bars that he went into. If he paid by card there will be a paper trail. Get him to put you in touch with his mates or whoever he was with in order to verify his story. If he's innocent and scared of losing you, he will be able to provide you with details. And don't buy his threats/complaints of "you should trust me". Do not let him put any of this back onto you.
Good luck OP.