krissy i agree if you are speaking on the terms of an endler, but i am pretty sure that guppy males fin is larger. i have both guppy and endler and there's a pretty big difference with the males, i can see how a female endler and guppy could be mistaken though. people please chime in here.... top photo endler male, bottom photo on the top yellow/black tail male guppy.
I have both Endlers and guppies. Male guppies usually have a larger tail and more colorful. I have had some colored females before that look male. You could also get a plain male that looked female. I find that gravid spot is an easier determination than the fin because it is easier to see and shows up earlier.
As to what to do with your little guy, I would suggest getting a separate "bowl" (large tupperware dish, etc.) and putting him in there. If you can put in a plant that would help destress him. Add aquarium salt to the water will help detox him. Salt is awesome for livebearers. You can add it to your other tank as well. 1 tsp per gallon if you have live plants. 1 tbsn if you don't.
Leave your guy in the separate tank for a while. You can use tank water to change it to keep the quality the same. Then try introducing him back.
They could be picking on him because he is sick. Animals do that sometimes. Once he is feeling better, they may leave him alone. If they don't, you can choose which ones to take back to the lfs and trade for fish who will get along.
I have kept 4 guppy males together without a female and didn't have any trouble. So it is not impossible.