He should start a conversation as you approach. Maybe save the game under a new name (a temporary save), and try the workaround for the now-fixed bug with the cure ingredients (finding and moving them in your inventory before speaking to the alchemist, causing a quest entry to be updated prematurely, so a cut scene was not triggered).
[*] If you have a save before finding the alchemist's ingredients and moving them, copy the 2 story.* files from that save into your latest save, otherwise you can use
these files from a save at the beginning of the imp village.
[*] Talk to Altoflix in the north of the village (if required), then the chieftain's son to get the quest to find the alchemist, talk to the alchemist once (only) and head to the cave.
If this works, you will have to reload a previous save, complete all other quests, and then perform this procedure again (changing story files may interfere with completing partially done quests).