What is a common blessing given to the bar mitzvah boy when attending his bar mitzvah?
What is a common blessing given to the friends and family of the bar mitzvah boy when attending a bar mitzvah?
Mazal Tov, may you give your parents much nachas, May you have a lot of nachas from your son, In Chabad: may you grow to be a chassid, yorei shomayim v'lamdon
Or you can make your own unique brachah. No one's judging you
Keep in mind that there is no halachic requirement to make any type of celebration for a Bar Mitzvah. The day comes, and, the boy is now obligated to perform all mitzvoth and becomes responsible for his actions. Technically, no one needs to even "cheer" about it in any way, though, of course it is a nice action. That's from the halachic view.
In my shul, the Rabbi does the following:
He reads a blessing that I believe is found in a book called "Hamdrich" a.ka. the Gabbai's Guide (I'll try to get you the exact title and publisher in a day or so),
Essentially, the blessing wishes that the boy mature in Torah learning and the performance of good deeds. He should be an asset to his parents, friends and the Jewish community. He then concludes with the Priest blessing (from Bamidbar).
There is no blessing to the parents. I think that the blessing given to the Bar Mitzvah boy, esp. the part that he becomes an asset to the parents IS a blessing to the parents, in a sense. However, the rabbi asks the parents to say Shehechiyanu.
As for the friends, family and guests, as well as the Bar Mitzvah boy and parents, the rabbi makes 3 blessings for them to which they answer "Amen":