I have a betta named Milhouse and I added 3 ghost shrimp in my tank. What happened was the opposite he owned all 3 shrimp and ate them. He's pretty badass.
If you want his fins to grow back ASAP add a tablespoon of almond leaf extract in your water per gallon (this can make your water brown which is natural if you don't want to turn your aquarium brown transfer popcorn into a hospital tank). If he's not used to it I think its the right time that Popcorn should get used to Almond leaf!
This is what pros do to condition their bettas in Thailand before a fight and it has a lot of natural vitamins and minerals for betta fishes. This is a natural remedy for sick bettas so don't waste your money with fancy brands. If your betta has a parasite this can be the best defense.
I also have a Bruce Lee Plakat ( I call him Bruce cause he's black and yellow like the jumpsuit that Bruce wore) He's tough like a brick and very aggressive.
I was transferring him to another tank and he got stressed out and next thing I noticed is that his caudal fin was torn. Only a tiny bit but still noticeable. I Added a tablespoon of the extract and the next day I was amazed his fins are back in shape and it only took 1 day! Yes only 1day! Try it out this might help. Hope popcorn becomes an awesome warrior like the Bettas I have
for more info about Indian Almond Leaf check this out:
you can also let the leaf float on top of your aquarium it makes it more natural and when it sinks just leave it the minerals go into your water and do its thing. My bettas also uses it as a bed at night which is cool!