Boy I sure know that one. When I first started with Muscovy I had 3 females that would refuse to go into their coop and would get up on the steep bank behind our home where I could not get to them Finally I figured out if I blocked this off and put food where they had to walk into a smaller area...
If he is completely dry and acting normal other wise eating and drinking and his coop is dry . He would probably be okay to go back out. I'd keep him out of water till it gets past these frigid temps we're having them too just above zero for lows teens and 20's for highs. My Muscovy are 12 yrs...
What your doing is good. Muscovy just don't have as good of water repellency as other domestics I have noticed it with mine.
@Jenbirdee has given good advice let us know how he is as he warms and thaws.
I sure agree no need to put water and feed inside for the ducks. I do put food and water...