Make sure you give him a moist place to shed. I got mine one of these (with some wet moss) and he loved shedding in there. Since then he has been eating better as well.
https://www.amazon.com/gp/product/B01N1EOR0K/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
Have you tried shredded aspen bedding? It's super soft and holds tunnels extremely well. I use it for my 6 sand boas and I like it so much more than regular aspen I'd never go back. You can get it at pretty much any petstore or online. I'd imagine paper bedding would make shedding difficult for them bc of humidity. https://www.chewy.com/zoo-med-aspen-snake-bedding/dp/123799?utm_source=google-product&utm_medium=cpc&utm_campaign=12868174255&utm_content=Zoo%20Med&utm_term=&gclid=Cj0KCQjwuMuRBhCJARIsAHXdnqOx7KVICRs1t7wkbnKcyqCczH_K964b06iYq9y...
I have the 75 watt version and 150 watt version of these heat lamps and I like them both. I paid $9 for the 150 watt so a good deal. At pet stores you're gonna pay about 2-3x that price. https://www.amazon.com/dp/B0002DHODG/ref=cm_sw_r_apan_glt_i_B3WEAAC4YX28S05DS0ZG For a thermostat I've been using this one for about 8 months and I love it. It's kind of hard to program but once you figure it out it's super easy. You can have two different heat sources hooked up or just throw the second probe in the same tank to read the temps. Try a CHE instead of an actual basking bulb like someone else on here said. https://www.amazon.com/dp/B083Q7YRBM/ref=cm_sw_r_apan_glt_i_B4SKD5ENMRRS3VF0H06T?_encoding=UTF8&psc=1
Do you have a humidity gauge and a temp gun? All reptile owners should have a temp gun for super accurate readings for different parts of the enclosure.
Unfortunately, those temps are waaaay too low :( ambient should be a minimum of 75° and basking spot should be closer to 100°. Where do you have your heat mat probe situated? Heatmats don't penetrate the substrate well and often aren't adequate. I'd highly recommend getting a CHE (ceramic heat emitter), or even better, a DHP (deep heat projector) in order to raise the ambient temperature. Then put a piece of slate directly below the overhead heating element to make a nice basking spot that naturally retains heat. My KSB doesn't lay on top of the slate like my other reptiles, but he has dug himself a nice little cave directly under the slate and that's where he spends a majority of his time ❤️
Regarding the substrate, I don't like the coco fiber as I find it to be very dusty and holds moisture too well. Personally, I use a 70% topsoil + 30% sand mixture, but if you don't want to go that route, I'd recommend aspen shavings over coco fiber any day.
Bumping the temperatures up might resolve the mouth rot on its own, but if you're able to, going to the vet for a check up and antibiotics is obviously preferred.
Reddit crashed while I was writing up my comment and I had to type it all up again, so sorry if it seems curt or I've missed anything. If you have any questions please let me know.
Here is a link to some cheap and accurate humidity gauges: https://www.amazon.com/dp/B07QMZL448/ref=cm_sw_r_cp_apa_fabc_V4YWMY7BG41QQTGVCVH6?_encoding=UTF8&psc=1
And a very good temp gun: https://www.amazon.com/dp/B00837ZGRY/ref=cm_sw_r_cp_apa_fabc_287D3S4XF635V8051SA1