In the brooder he may be too hot or cold, he will move away from the light if too hot, or directly under it if too cold.
Make him a mash with boiled egg yolk and unmedicated chick starter and lukewarm water. Put a drop on or inside his bill. Show him where the water is by putting his bill into it a couple of times and trying to get him to drink.
The other thing - you could cuddle him by wrapping him in a wool blanket and holding him against yourself, this would also keep him warm enough and he'll be happier to snuggle with someone rather than be all by himself

you might consider getting him another duckling. chicks and ducklings should never be alone. they always need at least one other play mate to help incourge them to grow properly.