I don't know if you'll ever be sure of the breed, but you can still take proper care of him. Care for chickens is the same for almost every breed:
He'll need water and chicken feed (chick starter is fine for his whole life, or he may share the ducks' feed). He'll need a place to get out of wind/rain/snow and to sleep--maybe a roost in the ducks' house, if he continues to be happy with them. He'll need to be safe from predators--inside your house or the ducks' house will probably be fine. Since "he" appears to be a rooster, he will not need a nestbox, "layer" feed, or oyster shell.
(Chicken breeds that need special care: silkies have a strange kind of feathers, Cornish Cross get too fat and die, crested breeds often can't see because of the feathers falling over their eyes. Your chick is not any of those.)
Have fun with him!