How does a mother write a speech to her son being bar mitzvahed?

It depends on the tone you are going for.

If you can pull off humor, I would recommend that route. Think of three funny, but not embarrassing, stories about your friend that show the kind of good-natured character he has. Then finish by saying how proud of him that he managed to fool his parents into thinking he cared about anything more than the party. That should get a laugh.

If you can only pull off serious, then give one or two stories that show your friend choosing to do what was right over what was easy and how you support his decisions.