Whoa, mom, calm down. This isn't as bad as it seems. You are not losing your son.
What you are experiencing is pretty natural and normal. You've been lucky that you've never experienced it until now.
1) Your son is going to have to be allowed to do things his way. You can suggest a check register one time, but after that you have to be quiet and allow him to use his phone, if that's what he wants to do. If he gets overdraft fees, he will learn. It took my daughter being charged $100 in overdraft fees for her to learn. I did NOT rescue her by paying the fees. She had to feel the pain of losing $100 before she figured out a way to manage her bank balance (I still don't think she keeps a check register, however).
He's been sheltered, so now it's time for you to back off and let him learn how to be an adult, and he will make mistakes along the way. You have to stop hand-holding him to his job -- if he was an honor roll student then he is certainly capable of handling his job without mommy taking over.
In brief, you HAVE to allow him to be an adult and make mistakes. So stop mommying him.
2) Set your rules and boundaries. If he is going to live in your home, then he must abide by certain rules. Those are:
He is not allowed to be rude to you.
He is not allowed to be rude to his grandparents.
He will pay for X, and he will not argue about it.
He will be courteous, and say thank you when appropriate.
He will do X to help out around the house.
And those should be your rules for him. Other than that, let him start making his own decisions. You have been probably just a little too involved, but I understand that it's hard to know, especially with your oldest, when it's time to back off. But it's time to back off, now. And only rescue him from any situation once, and only if he's polite about it.
You write, "all I wanted for him was to do the right thing and not mess things up." He's not messing up, he's a young man beginning his journey, which will be paved with numerous mistakes. And mistakes are great learning tools.
You write, "my fear is if this gets too ugly, he'll never bother with us again."
Don't worry. Even if your son becomes more remote for a while as he learns how to be a young man, he will definitely come back to you (unless you can't back off and let him be an independent person). But you have to give him this time to separate from you. Separation from their parents is a necessary part of their growth, so that they can become self-sufficient and someday bond with other partners.
If your son is unwilling to abide by your rules (which should be something like the ones I suggested above), then help him find a place with a bunch of roommates, and help him move out. Kids learn SO MUCH, so quickly, once they are on their own -- I personally think it's a blessing if you can send them on their merry way at 18. That's one of the best reasons for college, even if they are pursuing a degree you may not think is relevant.
Setting rules and boundaries will not make your son hate you (although after reading the other responses, someone pointed out that your kids should hate you at least three times. Very true!). He might "hate" you in the moment, but that's not permanent.
However, you have to stop micromanaging him. And if he keeps being outright rude, like blaming you for his gas problem? Then find him some roommates, mom. No 18 year old young man, who is old enough to go off to war, should be sitting around the house blaming his mother for his lack of gas in his car.
There will be growing pains, but it will turn out fine in the end.