Sounds like you are where I was with my nephew-son as I call him. My sister brought him into the world but I raised him and rescued him from her later in his life.
I was a big enabler always coming to the rescue. I would push finding a job going to school but as long as he had the convenience of a roof over his head, lights, food, running water and cable tv there was no motivation on his part. It took about 3 years from 18 to 20 before he finally got straight and he had some bruises in the course of those years due to some bad decisions he made.
But I finally removed myself from the equation. I am not providing transportation, when the food runs out of the house the food is gone unless he has a portion to contribute to groceries, I gave him 30 or 60 days to have a job and then another 60 days to get his own place after he started working. That was that.
I know in your case it is a little different but he is not going to understand until he is out on his own and as mother-in reality- you may eliminate some of his extras but you are still going to provide those necessities no matter what he does. It sounds like he was a high achiever in high school and may have had a lot pressure on him and right now what he is experiencing in freedom/relief and he realizes it and he enjoys it. It does sound like he has a plan it may not be the plan or course of action that you want him to take but he has a plan. From personal experience when dealing w/boys they do the exact opposite of what momma says "just because" but they always come back to momma. I would suggest taking the approach of asking him what are his plans for his life, education, work, and survival. Just remove what you think he should do from conversation all together. Then have some real talk with him. Ask him how would he survive if you were killed in a freak accident today? I know most people don't like to talk about this but reality is it happens and should be discussed. You will have to change your approach to reach him especially since he has a girlfriend he is chasing. Do not appear to be controlling him or "telling" him what to do.
For a budget you can do this yourself with pen and paper. The two of you together sit down and write down everything you pay for him and give him directly. Then write down how much he brings home from his paycheck and what he spends it on. Using the 2 list assign him some responsibility even if it's only $5 or $10 then you can take that money he gives you and put into a savings account for him, so when he does leave for college and calls home for help guess what, he has some money saved up (it may not be alot but it is something) and plus it will be like gold to him because he doesn't even know he has it. I would also have him put away an additional set amount for his personal savings it can be $10.00 a check. The key to learning to save is to start small and get used to the process and then gradually increase. When he gets a raise take that extra money from his raise and put that directly into savings. If you don't think he will be responsible enough to do it own his own have him give you the money or set his checks up for direct deposit and have the money automatically transferred to savings or log on and transfer it for him. Also you can monitor his activity until he understands some kids need a little extra guidance than others. I had to do this for my niece when she moved with me and started working. That girl spent a 100.00 in 3 days buying junk. After I showed it to her in black and white and started monitoring her account she became more conscious of what she was spending.
Last but not least PRAY about it and ask God for guidance for yourself and your son!