Does your son still live at home? (It seems like from the car example you mentioned that the physical location of his job was relatively close by)
Is it possible that your husband disagreed with your son's plans for post-high school? (You also mentioned that he works hard but did not mention any education plans. Maybe your husband wanted your son to go to college but he opted for a job right out of H/S instead.)
You also made it clear that your husband did not agree with his choice in car. If this is true, that would make another example of your husband having given advice that your son ignored.
If I'm anywhere close in my guesses, and these aren't the only examples of your son ignoring his father's advice, then it seems that your husband once looked upon your son favorably but began to take personal offense when your son began to ignore his well-intended advice. Being men, they'd rather thump chests then do feelings and over time has turned into a passive-aggressive conflict. Now your husband either has a "He made his own bed, let him lay in it" or a "I wasted enough of my time trying to help him" mentality. Neither really helps the situation, but eventually, your husband and son will have to work it out. Let them. They are both adults. But you need to remember that you cannot force people to like each other. Don't be disappointed if things are cool between them for awhile. And at the same time, instead of arguing with your husband, sometime gently ask him if it has anything to do with him feeling slighted and listen to him. Remember that your husband is allowed to feel frustrated if his son chooses paths that he feels isn't right. Maybe even let your husband vent a little about him to you - it may prevent a blow up between the two of them later.
Another perspective about the car too: I don't know what your financial situation is, but I'm guessing that your son bought his car used, not new. If money was any type of issue when buying that car, it means your son had to prioritize what features he wanted. Fuel mileage, warranty, age, reliability, body/ paint quality, history of upkeep, etc. all come into play besides just overall price. If your son was more concerned about the stereo system than the safety features, part of me understands your husband's attitude (even if I don't necessarily agree w/ it)...