help with flavor for fitness drink

my fiancé has some powdered supplements that he takes for working out. they are very bitter and so he has been using powdered drink mixes to hide the bitterness. However the citrusy and lemon and even other mixes which contain cictric acid have caused canker sores.

I can’t figure out what he can get to make his drink tastes better but won’t be so acidic because no one wants dreaded canker sores.

i thought maybe swishing or adding baking soda could help temporarily