"...he gets irritated when he sees we're headed back to his cage."
Hi there!
Monitors are not like any other animal, they're in a class of their own. About the above statement - It's possible that your sav is getting irritated once it sees it's headed back to it's cage. However, I think that your interpretation of "why" it's getting irritated isn't accurate. In other words, if it does get irritated at that point it's most likely due to not wanting to go back into a container that it doesn't feel safe in because it is regularly being accosted (in it's eyes) and has no way to escape it.
Your sav's thoughts are most likely, "If I go back in that cage, there's no way to escape being handled, and I don't want to be handled."
In order to understand and interpret their behaviors it's important to know how they behave in the wild. Savs are known for playing 'possum, both, in the wild and in captivity. Some will lay perfectly motionless in hopes that their predator, including humans, will leave them alone. Believe it or not, this is the reason your sav "...just wants to hang out in my hand. He stares at me while I hold him in my left hand, reading over my records with my right for 2 hours."
Trust me, I wouldn't tell you something that's not true. Your sav is actually petrified, terrified, scared to death. Defecation is an ancient animal trick that a lot of animals use to discourage their attackers. Bearded dragons don't defecate when force-handled, monitors do.
You don't want to "tame" your monitor. To do so would be a death sentence for it. It's cage is it's security, or, should be.
"When I put him back in the tank, he scales the sides again until I pick him up, and then he mellows out again."
When you put it back in the tank it's trying to escape you. Once you reach in & pick it back up it realizes that it's no use and it goes back to being your "rag doll." It's afraid to try to get away from you, afraid that you'll kill it should it try to escape.
Here's some pics of a 2ft fresh wild-caught that I had purchased the day before these pics. I walk into the room the next morning and see it laying on it's side, looking as though it fell off a branch I had in the temporary tank and it looked dead. I stared at it for a few minutes and then saw it breathing so I grabbed my camera.
This is the first pic I took. Notice that it's eye is closed.

Here's the 2nd pic I took. Notice that it's eye is now open but it still hasn't moved.

I told my wife that it was scared to death & that it wouldn't move. She didn't believe me so I removed the branch without disturbing him anymore than I had to. Take a look for yourself.

I surely didn't train him to lay like that. Incidentally, the cage was clean the night before, and of course, after the photoshoot.
Take a few minutes and read every post within these two threads linked below:
forums.kingsnake.com/view.php?id=1511176,1511176
forums.kingsnake.com/view.php?id=1515631,1515631
This doesn't mean that you won't ever be able to interact with your sav, you just have to earn their trust first, and then let them approach you. They will eventually if you allow them to feel secure within their cage and they'll realize that you're meeting their needs. Let it come to you, don't go after it. You know how much more rewarding it is when animals, people, do things on their own instead of being forced to.
Have a good one!
HH
-----
Due to political correctness run amuck,
this ol' hillbilly is now referred to as an:
Appalachian American

www.natures-signature.com