It really is refreshing to see two young people trying so hard. Sometimes encouragement is all that is needed. That sounds simple but true. You really can't do anything about what your fiance's boss said but sometimes, bosses change their minds. A good testimony goes a long way. My husband is a successful businessman these days but a long time ago

we were in your shoes. 2 jobs, 2 kids, no money, going to school, and living away from family support. Unfair junk happens and you just have to walk through it doing the best you can. Most of the time, you arrive victoriously on the other side.
It's best, you and your guy just don't let it bother you right now. Instead, help him to concentrate on the future and keep your goals in the front and important. Time will take care of this and your character will be strengthened. I promise! It won't be the last time you get a bad boss either.