Riley picked up all of his kids from the dregs, right? My money is on Fred being a brilliant but underprivelleged child, probably from an abusive home. To preserve himself from angry people, he would have been trying to make himself invisible or not worth noticing, maybe repelling people by acting weird because he was afraid a relationship would turn sour like his home did. That is exactly what his power does. People can't stand to think about him, even when he's not around. They call him "Freaky" not only because he's mentally repulsive, but because he wants them to so he can simply be left alone to study/think/do whatever it is that he does.
However, that doesn't really explain him being able to open up to Bree. Perhaps, human-Fred wanted to be able to trust people, but he lived in such a bad place there was no one he could trust. So he avoided people, made them want to avoid him, and he could do his stuff in peace.
That's my theory.
I wonder if Bella would be affected by his power? The repulsion seems to be a purely mental, if extremely strong thing. Andh ow would his power be nulled by her shield?