In leo's i would probably be more concerned about parasites than i would about mouthrot, especially since you said his stools dont look normal.
Mouthrot is commonly caused by a respitory infection that moves up the respitory tract and becomes rampid in the mouth. Respitory infections are usually caused by tank temps being chronically too low.
I would definately find a herp vet ASAP, especially if your leo isn't eating and is losing weight. Many conditions can be simple to treat but they get much more difficult as the leos health declines...the longer he goes without eating and the more weight he loses the less chance you have of being able to get him back. So find a vet ASAP.
A little info about your set up would be usefull...
Is your leo housed alone?
How old is it?
How long has it been since it last ate?
How long have you notice the appetite declining?
What substrate do you use?
What are the temps (high and low)?
What is the humidity?
For now...if your leo is housed with others then seperate him immediately to a hospital tank. Make sure to wash your hands througholy after touching him in case he has something that is contagious to your other animals.
Set him up in a hospital tank on paper towels, with several hides, a water dish, a calcium dish, a hot spot of 90degrees, and ambient cage temps in the 80's.
Make sure to bring a fecal sample with you when you go to the vet as parasites may be involved. If the vet cannot get you in very soon (next day or so) you can try tempting him with slurries of chicken baby food with calcium powder mixed in.
Definately call around tomorrow to find a good reptile vet.
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0.1 Bearded dragon
0.1 mexican Black kingsnake
1.1 Leopard Gecko's
0.0.1 Tokay Gecko
1.0 Blue Tongue Skink
0.0.1 Reverse Okeetee Corn
0.1 Anerythristic Corn
0.0.1 Red Tegu
0.1 Bullmastiff
4.1 Cats