You told what is known as a “white lie” which is a small lie told with good intentions to spare someone’s feelings or because you don’t want to tell an uncomfortable truth. The Church teaches that such lies are wrong, and I had one pastor who preached constantly against white lies because he thought small lies would lead to a habit of lying, eventually telling big lies and committing big sins.
In this case where your intention seems to be to stay out of a bad situation and follow your mother’s guidance, it’s probably not a large sin, but still a sin. Please talk to your confessor about venial vs mortal as we should not be pronouncing on that here.
The underlying problem here is that the man may think you are still interested in him and keep asking you out. Some men are persistent that way and don’t take the hint. The best thing would be to tell him that you are not interested in dating him. If you can’t bring yourself to do that, then the best non-lying excuse to use is, “Sorry, I’m busy” or “I have other plans”, as you could be “busy” with virtually anything and “have other plans” to do virtually anything - taking a nap, washing your hair or reading a book. However, if he is persistent you may end up telling him you’re “busy” 20 times in a row and finally have to tell him to stop bugging you.