Sounds like the ulcer/mouth rot was from pushing his face on his cage to get to the females...? Can you house him where their scent won't be an issue?
This can be so frustrating but I hope he turns around for you...especially since he's only 12. Wish I had more ideas, but besides swabbing his mouth & the
antibiotic injections & soaking for hydration, I can't think of much else, unless you want to tube-feed him to keep up his energy (& presumably his immune
system functioning), & that also depends on whether or not food of any kind is advisable att (some antibiotics don't mix well with food & snake may regurge).