Whenever I hear someone is planning to drywall a tub surround, then use mastic to adhere the tile, I move into emergency intervention mode, because these are two things that should never happen independently, much less in tandem.
If you ask for tile mastic, that's what you will be sold. Thinset is not mastic, for cryin' out loud, even though it's used to stick something to something. In tiling nomenclature "mastic" is a specific material, not a generic term.
I was being coy at times, and opinionatedly emphatic. I need to mellow out more, I guess. It's no skin off my nose if he uses rabbit-hide glue for his tile.