Hmmm, while Fox said it partly in jest I think, graduating for a car upgrade isn't really the right reason to get a degree at all. I think that will come in time though.
Your son will probably find that in a few more years he wants more, not just more money but more from life, more from his work ...something to challenge him, I don't know really since I don't know him, but I bet he will. That's probably when a degree will seem like a good idea to him.
Having said that, I've started to feel this way now (I'm 26), however I already have a degree, which is fortunate as what I am considering doing now (you were right about the electrical idea I had, that was really just something I thought might give me an easy route to Canada, that's a poor reason tbh, thus I have binned that), is something I wanted to do years ago but shied away from for various reasons. But, having a degree already gets me half way up the ladder ...if I hadn't gone to uni when I was 18 I'd have a much higher ladder to climb now and I'd probably be less inclined to bother. If you are wondering what I am on about, it's law, going to law school is something I fancied doing since I was a teenager but for a host of reasons I wont bore you with, I didn't, really I think it all just seemed to difficult and to take too long when I was 18-19.
Of course the reality now is, if I quit my job and studied full time, I could crack what I need to be a solicitor within 2 years ...which isn't so long really, if I hadn't already got a degree, it'd take 5 or 6 years. If I started in 2011, I could be working as a solicitor if everything went to plan by the time I was 30.
I guess my point is that there are advantages to doing a degree when you're younger, since you might find you want to do some post graduate education a bit later on, and you might find you have a much shorter road into it if you already have a degree. On the other hand there are advantages to waiting and doing a degree a bit later, in terms of having a better idea about what you want it for and being a bit more mature with regards to getting it done rather than being preoccupied with other things that are new when you're 18. At 21 though your son should have gotten over that ..wow I'm 18 I can do anything I want stage by now, 21 seems like a good age to get a degree actually.
Still if he really doesn't want to, I can't see anything good coming from trying to force the issue.