Some who say a guy wouldn't introduce you to his mother unless he does like you. That depends upon what number girlfriend you are? It could mean just what you said he introduced you to his mother. Meeting Mom is no sure sign of anything. After all you like him, you'll have to learn to like Mom and likewise. The worst part about meeting Mom comes in when Mom got attached to the last person she met, this will make it harder for her to accept you. Humans don't like change, we're creatures of habit.
Its a good chance hes looking for his mothers approval depends on the circumstance but if you like each other don't let mom get in the way.