Yes spouses can change!
I'm a 19 year old, so I've never been anyones spouse, however I have seen my own parents go through this type of experience.
My dad was heavily into drugs, and even abused my mom for a time period, he wasn't worth **** to anyone and just complete filth.
He ended up losing all rights to me and my sister, and my mom took us.
Months down the road, he's gone through rehab and got a bad but better-than-nothing job to be able to afford to pay the child support. He's never disrespectful to my mom anymore no matter how much **** she gives him when they meet up, and now she's decided to give him a second chance. It's been over a year now, he doesn't touch any drugs besides caffiene and he's just so grateful to have his family back.
So yes it is possible, and sometimes people just need that leg up, in your situation your ex is living in his car, you allowing him to come back with you giving him that other chance, he could have the mind set of (Oh I'm not risking ******* this up again) Because in all honestly it's so much better to raise your children with BOTH parents. However if this is not the first time you've given him a second chance, and he's messed up multiple times, than it's pretty clear no matter what he'll just revert to his old ways.
However if you love him, and this is the first time you've considered giving him another chance, you might be surprised how much people can change.