Assuming he already has his RN:
Linfield is a brick & mortar school with an online RN to BSN, it cost me around $12,000. It's located 140 miles from Madigan Army Med Center. (If it matters/helps Linfield is based in Portland, OR where there is a huge VA Med Center)
Western Carolina is another brick & mortar school with an online RN to BSN and costs under $5000. Western Carolina University - RN to BSN Program No clue about Army hospitals being located within 100 miles but I assume so.
If he's starting from scratch, keeping it under $18,000 you're looking at doing pre-reqs at a community college and transferring. It's going to be very difficult to do the whole package for that price.
Your best bet is going to be to pick a state, find out if/where there is an Army Hospital and contact all the Universities within 100 miles to check their tuition rates and admission requirements then tailor the pre-reqs at a community college to fit them.
Each State's Board of Nursing typically publishes the pass rates of the schools in their state on their website.