Mollies and most other livebearers will establish a pecking order. They know that he's a male, trust me on that, but they'll display to each other, and chase other males away, because they want to be the ones to mate with any females! Even in a male only tank, they will do this and establish who is top fish.
But the bullying can get to be too much, especially since this is a single new fish, entering the territory of several other males. It's important to have lots of cover in the tank to provide hiding places, and to break lines of sight, so the picked on fish can hide away when it gets too much, and the bully doesn't constantly see him and relentlessly chase him. Lots of tall decor and plants, whether real or fake, is essential if you want to keep multiple males.
Can you share a pic of the tank, please? Can give more specific advice if we can see the set up.
Another thing you can try, which sometimes works, is to remove all the males from the tank, and while they're in a bucket/container (cover it with a towel to prevent jumpers) re-arrange everything in the tank, and add more decor/plants if at all possible. Make it look different, and to the fish, it's as if they've been moved to a different part of the river, and they don't have established territories anymore. Put the new fish back in first, then add the others back. This doesn't always work, since they're more about pecking order than guarding a specific area the way cichlids do, but it's worth a shot since it sometimes helps, and you can more consciously put the tank decor in a way that breaks lines of sight and provides cover.
