I had this similar discussion when I went up for a summer visit last summer. When I spoke with the cadre, they made it very clear to me that it would behoove any applicant to try and interview with their first choice, rather than a local school - just because of the sake of logistics.
From what the cadre told me, you kind of have to look at it from the human nature aspect. So just for the sake of the conversation, we'll use your schools you listed. Say your son interviews at VMI, and mentions that it is his 2nd choice. Well, his interviewer may not be as inclined to pass him with flying colors (even if he did well) - due to the human nature aspect.
On the flipside, if he does interview at the Citadel, and does well, his interviewer may be more inclined to pass him with flying colors - since he/she also knows that the Citadel is his top choice. See what I mean now?
IMPO, I believe applicants SHOULD make every attempt possible to interview at their top choice. For one, even if he does not get a scholarship, your interviewer will realize how far of a trip it was for him to come all the way out to the Citadel. In return, he/she may be able to help your son in the future in things such as receiving a campus-based scholarship. This shows how committed your son is to come to the Citadel and their Battalion.
Now to address your question of whether it'd be worth the trip, only you and your son can answer that question. Personally (and based off the first post), I would say go for it IF it is logistically reasonable. Depending on who is driving, if you/him are able to make a trip out to the Citadel without facing any hitches, then by all means go for it.
If the Citadel doesn't seem to work out, then I guess VMI will have to be the one. But, just as Futuremom said, make sure your son is prepared for the "Why are we not the first choice" question.
Now, based on your second post - I'd be more inclined to say VMI -- but soley based on that post. The one thing I've learned from these forums is that although receiving a scholarship is great, but you must also consider the cost of college, say if he ultimately is never awarded a scholarship, or God forbid loses his scholarship.
Best wishes.