Please, please get him screened for depression and any other mental issues immediately--while he is still living in your home. If he is 18, he is an adult and if he won't cooperate, you might already have missed the boat in terms of being able to make him get treatment. Once a person is an adult in the law's eyes it becomes a nightmare for parents to get that person treatment unless the person is willing and also is able and willing to STAY on treatment if it's needed. Your son may or may not be depressed but what you describe is full of huge, waving, neon red flags, from staying in bed to changing personality to a sudden rift with (or dumping of, or dumping by) longtime friends. I also agree that your son might be largely affected by his friends' moving on without him since he didn't finish regular high school with them--even if he's not clinically depressed, he could benefit from talking to a counselor or therapist about that.
The fight should have been the real wake-up call. He could have been arrested; he could have put someone else in the hospital and been arrested and sued too; he could have ended up in the hospital. A random fight "on the way home from work"? Seriously? I would be suspicious that there is much more behind this fight--was it with the old friends, was he jumped and mugged but doesn't want to admit it, could he be (sorry to say it but you have to consider this) looking for drugs and got walloped by a dealer or....?....A street fight is not the same as kicking over tools and storming around the house (though those aren't great either). This needs to be dealt with so he doesn't end up with an arrest record, or using drugs, or worse. And sadly, yes, even the nice kids who are polite and help around the house can use drugs, especially if they are self-medicating because they have undiagnosed depression or other mental issues.
Think about it -- he is on a downward track here, and though it IS good that he's holding down a job, you know in your gut that he needs help you cannot give him. A professional needs to get involved, and now. Approach it with care because if he balks and refuses, you have an issue; you can't drag him to help. Consider whether there are others to whom he might listen, if he doesn't want to hear from you that he needs evaluation -- maybe a dad, an uncle, an adult male role model of some type he respects.
One other thought-- be sure to praise him for doing his job, for being so helpful around the house, for his good manners with the family, etc. Keep up some praise, even for things that seem small. But meanwhile, prepare to talk with him and express just as you do here that you see changes in him, his overall personality, his energy level, etc. that concern you. And I'd have an appointment already set up with a doctor who can screen him for mental health. Please update us!