As far as being afraid of the reaction of people in his life upon returning, I know we don't really "know know" these people, but you saw his Mom and best friend at the Presser -- IMO, these are the people you come to when you need help or comfort. If we see them as very approachable and kind (which I'm sure we all do), I hardly think Derek would fear them being mad/rejecting him. I doubt he is afraid, in that particular sense. Maybe he's afraid of facing disappointment in himself though, if you know what I mean. Poor Derek, I think this is some internal struggle that nobody could have foreseen.
As for his father, I believe Derek did experience his father taking off one or two times, and I believe his father was thought to be somewhere on the spectrum of bipolar, but I am just going off my memory of the initial statement from his brother. I didn't want to overwhelm his friend with that...I mean, he was nice enough to share about his dating life, and I felt funny even asking about that! :facepalm:
Well, fftobed: for this girl.
IMO, his mother did seem concerned at the presser that Derek may feel it would be too difficult to return. Both she and the PI seemed to express that they were concerned he may feel like he couldn't come back and they wanted to assure him that he absolutely can.