I agree with miss lydia! Thermometers and water bowls ar important when it comes to brooding ducklings.
On another note, a word of warning!! that box won't last long! It'll get dirty and wet pretty fast and he'll outgrow it pretty quick. Are you planning on upgrading it soon?
Could you you send a photo? If he hatched that recently, he may just be sleepy still.
Since you assisted, did you hit any blood vessels or cause any bleed? That could cause him to lose a little energy.
Is he now in the brooder, if so, what is the temp in there?