OK. So on the one hand I don't think it's likely that the military people your DS is around are going to encourage him to reconsider his enthusiasm for gas-guzzling vehicles; as far as I can tell, plenty of members of our military share that perspective and do, indeed, associate "manliness" with big, flashy, and, yes, powerful vehicles. On the other hand, the military is also an organization that puts value on families and respect (and care) for parents, and you may be able to use that to your advantage. But -- I don't know who your son's male role models are, but the reality is that you personally are not going to be able to have a "man to man" talk with him to get him to rework his perceptions, and really, it's hardly surprising that a not-quite-16-year-old is concerned about how to "become a man" or that he's getting his sense of what that involves from our larger culture (including some of its bad sides), or the military (which, again, has its good and bad aspects in this regard).
Where I was going with this -- and really, it's unfortunate for this purpose that you're not living on an isolated woodland plot as a hermit, but what can you do? -- is to figure that to the extent that you can frame what you are doing, the changes you are making, as being ones that will serve your son as he moves into his military career by giving you (not him, you, keep reading...) flexibility and security, maybe it will be easier for him to buy into. I mean there's him getting into VMI and its costs and certainly that's part of it. But beyond that, if he is a military man, he is going to be deployed ... you are going to be on your own, and need and should have the financial security and stability to care for yourself while he is out of the country. You will be (rightfully) worried about his safety -- what mom wouldn't? -- and need to have things "in place" and smoothly operating, with backup resources (i.e. a financial reserve) before that happens. Right? Can you buy into this? Can you present it to him in this way? Can you talk to someone who's involved with his ROTC about this and whom your son respects and get that someone to discuss this with your son?
Being a teenager is hard, and I totally agree with the others who have posted above that (as you seem to sense) there is some obligation on the part of parents to provide for our kids and that part of what we are providing is a sense of security and stability, including in what our kids can expect from us. You're making big changes in that regard, and for good reasons -- but truthfully, the need to make those changes comes from bad decisions you (and perhaps others) have made in the past. Which isn't a huge deal, we all make some bad decisions, it may not have been apparent at the time that they were bad decisions, some may have been out of your control, etc. etc. But your son has been growing up (and making decisions, consciously and unconsciously) about who he is, and who he will become, and who his friends will be, and all that stuff, based on one understanding of what resources are available to him, and now, all of a sudden, you are pulling those resources away. And, as noted in the comments above, not because of any sudden and obvious crisis, but "just" because it has dawned on you that what you are doing is not sustainable long-term. Which, obviously, is an important realization and I don't mean to discourage you (I think what you are doing is great). But I do get your son's being upset about the changes you are making and I don't think his being upset reflects immaturity (given that he's 15!) whinyness, or lack of character.
Hope this helps.