It's canker ...... meds for it #1- Ronidazole(ronnivet-s) in water for 7 days / (i would think 'fresh mix each day') .
I would (First) clean out all the yellow canker(don't throw on ground/ dispose of it properly/burn) and apply an Oral Antiseptic liquid each day from drug store for sore mouth canker .
Also i would feed a soft wet mix food to him each day (using some of the extra fresh Ronidazole mixed water for wet mix food)
Clean his water , feed bowls ... don't put other birds in w/him or in contact w/him .
Fishzole / Fishmox / metronidazole is also good(3 names for it) .