This is a tough age - they are legally adults, but many are so, so immature that it's like dealing with a 13-year-old.
I'm glad that your husband took him for his license, as that's a step towards responsibility and independence, and partially removes a big barrier, which is transportation. That said, your husband and his son need to sit down and map out his next steps. He's had a few months to blow off some steam and bounce around, and now it's time to settle into a semblance of a routine as an adult, which means going to work, paying some of his own bills, saving money towards specific goals (school, an apartment) and helping him chart out his next steps. Having the "fun mom" for him to fall back on definitely interferes with your husband's leverage,but it is what it is. He controls what is allowable in your home and if you SS doesn't like the rules, he's free to go elsewhere.
As for SS and you, just be the bigger person. I know it's hard - my SD (17) has been a snotty little brat to me since June and was the deciding factor in my husband moving out of our house and into an apartment (with her). I'm trying to be nice and be the bigger person, even when she's totally rude to me and has contributed heavily to the literal breakup of my family. I have to constantly remind myself that at the end of the day, she's just a kid and in time, will probably understand and regret her words and actions (or not). It's a hard spot for your husband to be in as well. Know that this, too, shall pass and be patient with him as he navigates this tricky time with his son and his ex.