I haven't read the responses yet. I think it's good for you to just let your feelings go on here. I work in academia and let me first say, let the boy make his decisions. He needs to grow up, and he's having a lot of support, but I don't see it as positive. Nothing bothers me more in my job is when one of my students brings mommy or daddy in. They are a legal adult, they need to be the proactive one, they need to be the one in charge of their life, not mommy and daddy. (another story)
My opinion is daddy needs to talk to him. Tell him, hey, I heard you may be quitting the job I vouched for you. That is your decision, but don't think I'll ever help you get another one. (This is my personal pet-peeve, I hate recommending people that I know, and they end up giving me a bad name for the referral.) I do think your husband needs to make very strong boundaries with him, this will really help him in the end. As for grandparents, well, they should too put strong boundaries, but that's something they need to decide.
I also feel your pain with the car situation. I would be hurt too, I won't even lie. I'm glad his mom and him are getting along, but buying your way in someones life never works. Hopefully they can have a great relationship.
He's a grown man, even if he doesn't act like it. There is no reason you shouldn't be happy he's out of the house. Of course you will always love him and care about him. There is nothing wrong with being happy for you and your husband that he's not that much of a responsibility anymore. I think good boundaries needs to be made for everyone's happiness, even if he doesn't see it now. You want him to be successful, you've given him the tools, let him learn how to really use them.
Hugs going out to you!