UPDATE!

Thank you all for your help and feedback. It has now been a week since my last attempt at feeding, so tonight I had another try.

While he did not take the weaned rat, he did seem a little interested in it. He came up to it at times to explore instead of ignoring or hiding away, and at times got into a strike pose but didn't act further.

I thawed in the way that I had previously (sorry to Mr. Misha, I'm not exactly comfortable leaving the dead prey item out for more than about an hour in ambient temps). dakski I just saw your response so I will try making the adjustments, namely a slower defrost instead of risking cooking it with many short bursts of hot water. I hope this is in line with Mr. Misha's idea of a slower defrost. I appreciate the exact temperatures you use, those will serve as a great guide.

Here is what I have tried since last time:
1) Cluttered up his enclosure as seen here: https://imgur.com/a/U98hcYf . This image also provides a reference to his size (at least his head) compared to the prey item. Black paper has been added around back and sides.
2) Feeding technique. I tried a more appropriate mouse impersonation than I had to with the other BP that I had cared for in the past.
3) Feeding in lower light and hidden behind the paper as he can freeze up when he sees me at times.
4) A week with minimal handling (No more than once to weigh him a few days ago).

I have not bought a CHE quite yet as I wanted to give some other steps a shot before rushing out to spend roughly $100 CAD (for new).

On his first weighing (Sept 5th) he weighed 545g with my newly acquired kitchen scale, and tonight (Sept 8th) he measured 537g. To me that seems like a pretty decent (worrisome) drop. That's 8g in 3 days. Now part of this could be scale error as it was a cheap scale, but I'm not going to pass that off as an excuse.

I am getting pretty worried about him (that's just my character), but I have just been notified of another pet/reptile store in the "area" that I had no clue about before. I'm going to look into what they have in terms of live prey to see if I can get my hands on a live pup or weaned rat to keep his weight up.

My further questions for the community are:
1) Is this weight drop something I should be worried about at all or is this rate normal given the circumstances?
2) Any other suggestions for the time being? I'm going to look further into heating now that he has rejected his latest meal.
3) Do you think that he would have settled down by this point? Is the new environment still a relevant concern? (I will continue to refrain from handling him until he eats)

Once again, thank you all so much for your continued help and support.