I don't know if Muscovey males (drakes) are as aggressive towards other males as Mallards are. You might have to read up on it, unless some body els chimes in.
You could look on Craigslist for your area for exactly what you need. I don't think they would need to match his age, but old enough (big enough) to run from him if they have to, and old enough to stay outside.
As you may have heard, ducks and most if not all birds are flock animals and need to be in a flock of the same species in general, to feel secure and safe. Any aggressive behavior he shows may be the result of feeling insecure.