Major field of study isn't a consideration in most career fields in the USAF. Some, like scientist and engineer positions, do require a degree in a specific field (I think you may have to have some credits in accounting to do finance, too). However, pilots, air battle managers, missileers, navigators (CSOs, whatever they call them now), security forces, intel, public affairs, combat rescue and a list of others do not require any specific undergraduate major.
The better question is what does he want to do with the degree in Russian? If he wants to do intelligence, then it is useful to have the extra language, and I'm sure he could put it to use. He certainly won't be pigeonholed because of it, though, and he can be a B-52 pilot or an aircraft maintenance officer, or whatever the needs of the Air Force are at the time.
I was an International Studies major with an English minor, and on active duty, I was a missileer for the Minuteman III ICBM. My undergraduate degree had nothing to do with what I did on active duty, except that I had heard of the concept of nuclear deterrence as a policy matter. That and $1.50 would get you a cup of coffee when it came to the ins and outs of operating the missile system. The Air Force will teach you what you need to know to be successful at whatever AFSC (Air Force Specialty Code) they assign you.
Hope this helps.