Several weeks about the nueter, that male can still reproduce. There is sperm still available sometimes. Once everything is done and over, and male dog can still get horny when a bitch comes into heat. They can still tie with the bitch, but it doesn't hurt either dog, and no puppies should ever come from it.
He can still get a female pregnant for several weeks. It can take that long for all the sperm to be cleared out of his system. He can still mate after being neutered (as has already been stated) whenever he desires to but will not get a female pregnant and it will not be his #1 goal in life anymore when a female in heat is around.