My daughter chose a ryukin for her birthday gift on a "pick out whichever fish you want" trip to the pet store. He is living in an unheated 10g with a decent filter. I understand he will eventually need bigger accomodations and plans are secretly in the works for upgrades as gifts later, but for now he has plenty of room. I am worried about water movement. He has the filter and an airstone thing that is about 1 inch x 5 inches. I have heard (and can tell just by looking at him) that he isn't the swiftest swimmer. Will he be ok, do you think or should we do something less active with his water?

If I do need to lessen the water movement, what suggestions can you make?

PS- Is there any way to tell the sex on a regular day (ie not "when in breeding condition") ?

Ryukins are a nightmare to keep from swimbladder troubles. A diet as free from flakes as you possibly can will help as will low nitrate levels.
As for swimming they arent the best as youve noticed and i wouldnt worry unless he is being constantly thrown around the tank or stays in one spot for a long time.
You can get a gang valve for the airline to slow down the bubbles if needed and placing a tall ornament in front of the filter should break the flow up a bit.
Congrats on the new fish.