Okay technically it's a fish tank, however only one side is still glass, the other sides were replaced with wood about a week after I got him, with the screen top now being a hand crafted fixture with a single hole for his basking light.
According to my temp gun the basking platform is around 130º, and the thermometers give a 90º on the hot side and 80º on the cool side. Is the cool side too hot do you think? I'd imagine it's cooler in his burrow.
Humidity is usually around 55%, but I bumped it up to around 70% just recently.

He has half topsoil and half play sand for substrate.

He only got handled when I helped with stuck shed when I first got him, and I would pet him during tong feeding, which he didn't seem to mind.
He has not been very aggressive unless you get too close during a delicate time, ex. trying to clean dirty water while he's soaking, or when he's basking. In which cases he gives a little hiss. However I have yet to see a tail whip or attempted bite.

I don't know what is the matter with him, the setup seems to be right, and he is acting like the little dinosaur he should be, aside from this feeding problem. From my limited monitor experience most problems stem from the setup, but it seems to be all in order.
I've heard of a similar "lockjaw" like problem with someone's Black Throat monitor by the name of Skittles I believe, however I can't remember for the life of me where I read the story.

PS. Possibly unrelated but his siblings at the petstore apparently died from mouth rot, is it possible he could have mouth road that developed at the store and has eluded my detection? Would he simply be refusing to open his mouth due to pain and/or discomfort?
Would it be a good idea to check inside his mouth, or would that stress him out too much?

I'll get pics as soon as possible, my cord to connect my phone is damaged (thank you girlfriend's cat) so it might be a while before I can get them.