1.) Your temps are too low. Anything below 75 is too low for the cool side, and the warm side should be at least around 88-90.

2.) Your humidity is too high with temps that low. The snake doesn't appear to be in shed, so your humidity should be between 50-60 at the highest for right now unless he has stuck shed, which he doesn't appear to.

3.) Based on the pictures you've posted, this unfortunately looks like scale rot to me. I know it's a weird question, but does it smell foul? Scale rot at that stage usually emits a moldy/rancid smell. If that's the case, he needs to go to a vet ASAP.

4.) Trying to help a BP gain weight by feeding every 5 days at that size is not a good idea. You are overfeeding right now and he isn't able to fully digest the last prey you've given him by the time you offer the next. How old is he? I would feed him every 10-14 days, and feed 10% of his body weight. I know the urge to bump up their weight is strong, but these guys are slow healers, and it must be done on their time or it could lead to worse problems.

5.) You said he doesn't look skinny. If this is the case, don't compare him to others' snakes. Perhaps he is OK at this stage? Either way, space out his meals.

I would take him to a vet regardless, as long as you can find an experienced herp vet in your area. Until then, make sure you adjust the husbandry as outlined above until then. The rot is most likely due to a combination of your high humidity and low temps. An adjustment should help. Good luck!