Difficult situation this, and one which I will no doubt encounter in just over a year when DS1 goes from high school to college.
About two years ago, a sandwich bar opened up a couple of hundred yards or so from our college, and it's been a right nuisance ever since. I know that the college haven't been happy about it, as it has resulted in many students gathering around there every lunch time, not only eating junk but also causing disturbances and congestion on a narrow village path and road.
The car parking outside the sandwich bar is also causing alot of traffic build-up, but that's another story.
I think the only think I could do is simply make DS1 aware that I would not be happy about him spending his lunch money on junk when the college has a perfectly good canteen selling good food. However, I know that I won't be able to be with him making sure he buys the right stuff, and nor would I want to. I understand that he will probably try it for a while, but I know that the novelty will wear off.
We have always ate healthily at home, making our own food and cooking from fresh. DS1 enjoys cooking alot, so he appreciates where his food comes from and what is good and not so good. I don't think a little bit of junk now and again does any harm, but to be perfectly honest, knowing DS1 and his likes and dislikes, I can't see me having too much of a problem with his lunch choices - hopefully!
Good luck with this one noddyholder - I hope it irons itself out in the end.