What happens when a mate dies
An unbreakable mirror as soon as possible will help a bit, as will time spent with you.
He does need companionship. And it can be tricky, because ducks are individuals, and how relationships develop is largely up to them.
Still, we can help them. They need to be introduced carefully, slowly, giving them all time to get acquainted. They can really fuss at each other at first, and sometimes ducks can be aggressive and hurt each other.
But most of the time, given slow introductions, ducks get along, and like to be part of a flock, however small.
Sometimes drakes can become buddies - especially when there are no females. But, sometimes drakes will hurt each other.
I don't want to distress you, but I don't want you to be surprised or unprepared.
I don't think younger would be better than similar age, even perhaps older. That is a good question - whether or not age is a factor in them getting along.
My newest three adoptees are all under two years old, and the rest of the flock is beginning their seventh year. The new girls have assimilated beautifully into the all-girl flock.
You may already know, but there is a Free ReHoming forum as well as a Where Am I Where Are You forum organized by location.
There are also rescue organizations - that's where the most recent seven of my non-Runner ducks have come from. Even veterinarians sometimes have contacts for ducks in need of homes.
Edited by Amiga - 4/9/16 at 4:53pm
Can you split the pen so they can see each other but not touch? That's what I do with introductions.
Then if it looks like they are okay, I let them spend supervised time together in the yard. After a few of those yard meet-and-greets, I let them spend more and more time together unsupervised.
If they are both lonely I reckon they'll get along.
Please keep us posted. And if it was a predator, please make sure the security has been taken care of, because they return.