Hi, I'd like to help you with your question.
It sounds like your son just needs a break. Not all kids feel they are ready to pursue education right away. Some feel that time in between high school and college gives them the opportunity to grow and mature. He may have thought that college was ok at first but decided that it was too much, especially with a full time job.
He may also want the chance to test out his freedom and make his own choices. He is young and newly on his own so he may feel the need to try different options to see what makes him feel happy in his life.
Part of the issue may be that your son is a high achiever. He may feel that he is always under pressure to perform and he has realized that the pressure is getting to him (This is a trait of high achievers. They often put pressure on themselves to do well). He may have decided that before he finds himself in trouble of some sort, he needed to take a break. Academic pressure can be severe, especially for the highly intelligent.
I would not worry about your son. He sounds like he is taking care of himself. He has a good job and is doing well, as you noted. He is also very intelligent so he either will eventually either do very well in his job or he will chose to go back to school and finish.
If your son should show signs that he is having problems such as losing the motivation to care for himself, not socializing, not caring for himself or rejecting any of your attempts to help then you should try to get him an evaluation with a therapist. Otherwise, give him time to sort this out for himself.
I hope this has helped you,