As long as you don't create a reason to have ducking, it's OK to omit it. If you have no attacks that are particularly avoidable by crouching, then it doesn't help in combat.
You simply have to place the design priority around the fact that, he doesn't have it, and that jumping and side dodge are your means of escaping damage. When you properly design a game, you design obstacles around the function and form of your player abilities. If done right, when you didn't include ducking you don't ever need ducking. It's as simple as that.