Rejected Duckling Can he Be Re-united?

I had a hatch about 5 days-6 ago and got 9 new ducklings. About the 2nd day I noticed one little guy was a little slow, and not getting under mama. I managed to get him to her and under her without too much panicking on her part (daddy was a different story) and checked hourly the rest of the day. About 6 pm I found him laying on his back in the middle of the coop floor, about 2 feet from the others, he was cold and listless. I almost didn't see him because he blended in with the straw. I grabbed him u,p managed to revive him and now he seems to be the picture of health in a box in the house.

He's got a fuzzy chick toy for company that he likes to stand on but he must be awfully lonely.

Would it be possible to reunite him with his siblings? Or would his mama protest? I don't consider her much of a mother, she never managed to hatch one egg all of last year, this is her first success, and what a bizarre time of year she picked!

It's an uninsulated coop with one 250 watt red heat lamp under which she built her nest. It was thermostat controlled to turn off at 45 degrees and on at 35 but since the hatch I've left it on all day. Still, that little guy isn't going to make it out there if she just ignores him rather than deliberately try to kill him.

He's been in the house about 4 days now.

(Incidentally, when I was out feeding them this morning I found a hidden duckling body. It must have hatched late or early and didn't thrive, it's very thin and was on its back as well, buried under the straw. Still have the other 8 I knew I had.)