Bless Him. I battled thrush for 3 months after COVID. What worked for me was Diflucan 150 mg for 3-4 days and Nystatin Mouth Rinse (you do this about 4 times and day rinse and swallow). The mouthwash may not be
doable at this point with the G tube. Thrush is stubborn and it may take a few rounds of both of these meds. IF he takes a lot of Diflucan, you will need to have his liver panel done to check his liver.

Candida (Thrush) usually starts in the gut. Mine is from antibiotics/steroids. I presently have thrush from them
and I am taking Diflucan & Nystatin.

Keep mouth as clean as possible in his condition. If the corners of his mouth are cracked ask them for an antifungal cream for that.

Praying for him and you. Blessings....