First off, it is not ususual for a FEB final determination by the 4-star to take 4-6 months. Remind your bro that FEB findings are only recommendations, and the Convening Authority for the FEB and the MAJCOM 4-star are free to concur or nonconcur with that recommendation and determine just about any of the possible outcomes for the respondent. In other words, just because the FEB recommeneded your bro be retrained to a different airframe, that doesn't obligate leadership to do that.
Second, FEBs have nothing to do with retention in the service. For an officer to be kicked out, he has to go to a Board Of Inquiry (a.k.a. a "discharge board"), which is an entirely separate process from an FEB. Even if an officer were to lose his wings at an FEB, the 4-star would have to initiate an entirely separate process to start and execute a BOI. BOIs are a whole additional legal process that takes place in a courtroom with another panel of senior officers, etc.