The liquid oxycodon is easy, it is pretty fast acting too. The amount needed can be readily adjusted. Reactions to it are individual, at least with the amount I took it did not impair me.
Good luck with the PEG tube, make sure he uses it! That is what it is there for.
Re recovery of taste/saliva. That may never come fully back, which of course is a pity but in the big picture......

M