I would not worry to much about him being a single duck. Keep on putting him together with your other ducks and eventually they will become on flock. Besides that I would give him the option to go back into the wild, because that is where he came from. It may be painful to you to see him leave, but it is also good to know that it went to its roots living wild and free. He may even stay because he wants to.
I had once a wild duck and I loved it when she started to fly always coming back to her home. In fall she did migrate with the wild ducks. I was sad to see her go, but was happy for her too. We don't know exactly what happened to her, but I do think she is still around. There is a small pond next to our property, which never had ducks because it is seasonal. We have seen a duck there every spring since she has left. We think it is her, but have no way of telling for sure. This spring the duck showed up with a boy.
Anyway I would raise a wild duck again anytime. And I do love to see it being re-integrated into the wild. It is a good feeling too.