Albert; You're totally right that at points in their life, especially as babies, freezing up can be a fear response. It's just not something I would expect when the snake has previously been active and not super-defensive while being handled to suddenly start showing that particular response every time it's held. If it was doing that from the get-go I think that idea would fit better. In my experience, once any of my garters have gotten comfortable enough to be handled without problem, they show you they don't want to be held by nipping and musking because they know you aren't going to hurt them so they are focused more on "Hey, I don't want to be picked up right now!" than "Oh no, I'm about to be eaten"
My guess is this guy is probably just calming down to being handled if he's showing normal alertness and acting normal inside the tank.