Awesome on the heat gun! Also awesome on the thermostats, again! You are doing great so far on the "checklist."
If you just noticed it and last week you were away, it could just be a little dry for him.
Great if you can change the water daily, but also okay to change a little less often as long as he hasn't used the bowl as a toilet. Then I would change it immediately and disinfect.
Awesome he is eating F/T for you.
Rat pinkies are probably way too small.
See this thread. https://ball-pythons.net/forums/show...=feeding+chart
The chart is in the second post.
He should be on pups or bigger, just looking at the pictures, and how long you have him and when he was probably hatched. He might be a little thin and that could cause the loose skin; worse if the humidity isn't where it should be.
Urine looks like pee, wet and yellow, and urate are solid whitish poop looking things that often come out with the urine. You might miss the first, but likely would not miss the second.
Address the hot side temp, keep humidity where it should be, and start feeding bigger meals. I think everything will be okay. I saw your other post about possible mites. I think you are being careful and cautious and want the best for your new guy. That's all good in my book.
Keep it up and don't be afraid to ask questions and keep learning. We don't bite!