It's OK, at 18, to not know how one wants to spend their lives. And, even more importantly is to know that any decision made at 18 can be changed along the way. What is important is to make a decision. That decision is only a starting point.
I suggest you help him by guiding him in ways to get information so that he can make a decision based on facts, not feelings. Of the three choices, going to a 4 year university/college can't be done this year. The second, enlisting, is a four year commitment. Junior college is the only choice that allows flexibility. Or, a fourth choice, working this summer or fall, also provides flexibility.
I suggest he's not ready to make a long term commitment. So focus on what he can do now and leave other options/choices to the future. Often, kids this age do not know what they want to do long term. Take that out of discussions.
Stop the nagging. You can't make him make a decision. Instead, focus on what you're willing to accept. Can he live at home, doing nothing. If not give him that information/boundary. Will you pay for junior college or college? If so, under what circumstances? You make the boundaries based on your needs.
It's very true that his decision now only needs a short term commitment. Emphasize that he is not deciding on the rest of his life. I went to college and became a teacher. After three years of teaching, I became a police officer. After 13 years as a cop, I wanted to do something else. After realizing that I would be able to collect my pension in five years, I stayed in law enforcement. After I retired, I worked at jobs, not careers, in which I was interested. I have a daughter and grandchildren. My life and interests changed over the years. We need experiences to help us make decisions as we move forward.
I would choose junior college and working if I were in his shoes. I'm an active person. If I were less able to maintain the energy to do both, I would choose just one with which to start to gain experience, to have time to mature.
I worked full time for a year before college to earn money for college. I had a goal. I knew my next step. He doesn't have to know his next step. He just needs to make a decision for now, perhaps for a year. Take the heat out of the decision by knowing it's short term.
Talk thru the choices with him. Emphasize that this decision is short term, give him deadlines based on what is acceptable to you and then leave him to make the choice.