A bit of factually unfair reporting/spin. A pilonidal cyst is not a boil on his butt. I had one myself for years as a late teen/college student from a fall from a step ladder onto my sacrum/coccyx. The cyst is not a boil - it can be a fissure and open up and drain fluid and some blood periodically. Some pain and discomfort, and can be a physical fitness issue for enlistment in the military (we want only our best young people, right?). It could feasibly be a performance issue I imagine or at least was a demerit on the physical fitness checklist. When my college deferment was up upon graduation, the Army had my name waiting for the draft physical and probable selection. I reported for the exam and was given a 6-month medical deferment because of the pilonidal cyst I had.
In the meantime, not wanting to be cannon fodder in the Army as a draftee, I managed instead to get into the Air Force (despite the cyst or because mine had at least gone into it's non-active cycle and I passed the USAF physical exam) and accepted into Officer's Training School (OTS) at Lackland AFB in Texas. I eventually retired from the USAF after 23 years (and still have remnant scars at the top of my butt crack from the cyst....)
I'm not sure why Rush didn't even know he had a pilonidal cyst - but it was/can be a valid factor in barring military duty (or was back when).
It's not a factor of "walking around would do him good" - it's a surgical procedure needed to remove such cysts, and back then it meant a long stay in the hospital on your belly, butt up and exposed to dry the wound out while the surgery healed up.
Modern medicine might well have a quicker and better way to deal with these....but back then it was a real enlistment/draft issue.
Otherwise, yes, Rush is or deserves a boil on the butt.