Hello, and welcome to BackYard Chickens.
Integrating ducks can sometimes be a challenge, but it's important you bring the duck out every day, or more, to get him used to the other ones.
If you've ever had chickens, you probably know about the no-touch but - see method, as the name states, it's a method of integration where the birds can physically see each other but aren't able to touch physically. Do this method for a week or so, then try letting the bird out supervised with the other ducks and see how they get along. There will be some bullying at first, but if it starts to get to the point where someones getting hurt, you'll need to separate.
To decrease the chances of them fighting when there with each other, generally, the more space you provide the less likely they will fight. Sometimes they'll fight over food/water sources so adding several feeders and waterer may help. Feeding them all treats when you let the duck out may bring the other ducks attention to the treats rather than the duck itself. Integrating them can be hard, but it can be done, so far I've integrated around thirty birds here with pretty good success, sometimes it takes lots of patience and time.