60 degrees and wet at 1 month is too soon. They don't have their adult feathers yet that carry the oils that will keep them dry, it takes like 3 months. The feathers will come in when they come in regardless if you take the lamp away or not, the only thing you'd be doing is taking away their heat, which isn't really good. He may have had a seizure, I wouldn't know. I do know ducks can live a long life with untreated seizures though(unless they seizure in water and drown). Keep an eye on him, a chill can kill a duckling(but so can heating him up too much, which is why it's best to put a heat lamp in the corner so they can go back and forth as needed), he probably just chilled quicker than the others since he is a little weaker, so I'd let him play in warm water for a short period of time inside instead of outside with the breezes. Also when they do get their feathers, they still may not have the oil. You'll know when they have the oil on them when the water runs off the feathers as if the feathers were plastic or polyurethaned wood. That's when they can be in the water without risk of getting chilled.