Just be there for him, be understanding about the time commitments and sometimes crazy schedules, and don't ask a lot of questions he can't answer. It's not personal if he doesn't tell you something, so don't be upset - basically, just avoid asking too many questions so he doesn't have to shut that door in your face.
The parts of his organization that are for public consumption, when the time is right, he will be proud to share, and MOST proud to share with the woman he loves (that's you!
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The times he is away from you, spend time working on YOU. He is doing something positive for himself and for others - time for you to do the same!