Thanks for your question! Since Stephen's been actively working on it over the course of a few months, there's no need to spend AP on it. You could assume that he, with or without help from other magic-users in the city, managed to make the potion you mentioned.

A body mod to make him completely free of needing to eat cosmic horror-terrors would probably be something that would have to be purchased through AP since it would be a physical change, but just a potion to make him capable of eating real food is totally fine to get through plot.