You're simply fighting a survival skill.
He trusts your break dancing hand but doesn't trust being held, yet.
Most garter snakes I've encountered learn to trust rather quickly.
I think you're on the right path with what you are doing. Try holding him just above the floor of his enclosure.
Don't take him out all the way. Let him crawl through your hands. Let him explore your hands and arm.
This whole trust thing has to come at his speed. You can't force it or he'll feel threatened.
Best of luck.