Hmmm, it's quite open.
Male livebearers can pick on each other sometimes, especially when there are no females around to distract them! Not that you should add females unless you want/are prepared for breeding, since livebearers produce so many fry, and a male only tank can work, it just takes a bit more planning and thought, and monitoring for bullies.
Firstly, fill the tank with more plants, driftwood, things that break lines of sight and providing hiding spots. You have a lot of that lower down, but the top is very open and exposed, and the bully can spot the smaller male easily, so he's more likely to relentlessly chase, you know? I'd add some floating plants with long roots like frogbit or water lettuce, these provide some dense cover, and fish tend to feel safer with overhead vegetation too.
I'd add some big bunches of live plants, some hornwort, elodea or water wisteria, some fast growing stem plants like that that can be left floating in a dense plant mass near the surface, or planted and allowed to grow tall, and that would provide cover throughout the whole height of the tank, you know? Or vallisenaria... grows tall and can be cultured into some pretty dense curtains of plant, great for hiding spots the whole tank height. Taller pieces of driftwood or stone can provide some additional structures with height to help. Live plants provide some environmental enrichment for the fish, since they interact with live plants in a way that they just don't with fake ones, and they help with water quality too, win win!
Will use my own tank to demonstate what I mean, so I don't have to steal some random photos fromgoogle. This tank is less than a month old:
So it hasn't fully grown in yet, but see the top right? The wood, some vallis, and the frogbit already form lot of hiding space there, and breaks line of sight between fish, so the bullied one can get away and get some peace, and the bully is more likely to lose sight of him and forget about him for a while, or get distracting eating algae from another plant. Doesn't necessarily have to be real plants either, the main point is to have structures of some kind that will break up the tank into areas, and give the fish places to hide and explore, alleviating boredom and reducing the harassment.
A couple of young platy fry, hiding out in the frogbit roots
If doing some tank renovations doesn't help, then adding a couple more males might. Since there are only the five fish in a 20 gallon, you have room for a few more, and it could disperse the aggression and keep the bully busy, so he isn't so focused on pestering the one fish.
If that also fails, try removing the bully male to a temporary holding tank (a bucket with an airstone and kept to the same temp and covered with a towel to prevent jumping can work) rearrange everything in the tank, and leave the others in there for a while, let them explore and establish new territories. Add the bully male back in. The idea is that it will look like a new area to him, like he's moved to a new area in the river, and it's not his old familiar territory anymore. He's more likely to find himself feeling like the new guy, will have to establish a new spot in the tank as his new territory and they others already have theirs, so he has to find his own, and it's not the whole tank anymore.
This sometimes works - African cichlid keepers use this method a lot! But, it doesn't always work either.
If all of those fail, then one or the other will need to be moved out permanently.
also keep an eye on how the others interact with the weaker male. If all of the fish are picking on one fish, that is often a sign that there's something wrong with the one that's being bullied. Fish torment a sickly fish. Sounds mean, but sick, struggling or dying fish attract predators. It's in a fishes nature to drive away another fish that is sick, since it could draw a predator right to them. It's another good reason to move a sick or dying fish to a quarantine tank, not only to reduce the chances of spreading the illness.
