Talk to him, give him the items he asks for and let him make the antidote.
Talk to him again to get the newly made antidote (you will not get the physical item).
Now, you have to go bring the antidote back to Sloutii. There is only a 1/5 chance that the antidote can cure him. If it fails, go back to Father Arthur to get another antidote (you will NOT need to collect the items again).
Go back and forth until you can cure Sloutii and he will teach you the skill.