According to the National Institute on Drug Abuse: while Kratom isn't an opioid, it can cause "opioid-like" and "stimulant-like" effects in the brain by activating "mu-opioid receptors"

Someone made this video that's a pretty decent watch

https://youtu.be/TLObpcBR2yw

technically no, but essentially yes

Yep, it's method of action is different but the end result is the same

edit: if he's still on powdered form, I can't recommend a short break enough. Even if it's just a week or two. Would be a good time to heal up the liver too.
If that doesn't work, slowly (and i do mean extremely slowly) lowering the dosage could work.
Different people work differently, some quit cold turkey and prefer that to weaning off. That is if you're in contact and he's willing to do it.