maybe, although he never actually tells you what he puts into the coffee, he just says that he makes a special coffee with a secret milky ingredient 😉 and Vicoria says he puts tar larvaes in the coffee.
Also he never said that you put love potion into the coffee, he said that you put something into the coffee and you could get in trouble because he doesn't know how is going to affect him. If Xeroth found out about the love potion just by looking at the tea, maybe Feliar knew you drugged by the taste of the coffee, been he a coffee maniac.
Now about the resistance to love potion it could be because he is a shapeshifter or be a Satyr but there's also a wild theory in this chat that Vicoria and Feliar were a couple before and she probably stuffed him with aphrodisiac so he may gain some resistance.
Think about it when Feliar and the MC ar fucking he cums 3 times and he's suppose to be an Old Satyr so a young one must be able to do it 5 or 6 times that is hardly enough for Vicoria so is most likely she use her powers or stuffed him with aphrodisiac to keep going. I mean we think Arion is the one that could satisfy Vicoria because he can at least cum 10 times in a row, when Mc is getting fuck by Arion in the stables he can get fuck like 9 or 10 times before black out and while the MC is a twitching mess Arion is like "yep that's a good hole, Next!"
So yeah is a little tricky to think that Feliar is the shapeshifter because he could be or he just could be an old Satyr with deep scars and trust issues.