Just watch him to make sure he doesnt display signs of being too warm- but being on his own and not having another duckling or two to snuggle up to- warmer is better than being too cold. If it is too hot- it will act very sluggish and try to get away from the heat, and want to drink ( more...
Looks like the yolk has absorbed- but just some veins there still to reduce and the belly to close up. Let the duckling rest and as it dries up- the belly will close up on it own as the blood leaves the veins. Just watch for infection in the guy. He could easily get an infection at the site...