Is it ok to have a Bar Mitzvah party before the boy puts on tefillin.
Mazal Tov! May you have much nachas from your son.
Regarding your question, the Bar Mitzvah celebration is because now the boy has the unique opportunity to perform mitzvoth and the will of God as an obligated Jew. (A Jew that is not obligated to do a particular mitzvah receives less reward.) Although the bar mitzvah boy puts on tefillin when he becomes bar mitzvah, (or a little before becoming bar mitzvah) the actual bar mitzvah celebration isn’t in honor of the tefillin.
The bar mitzvah party can be made before or after the actual day that the boy becomes bar mitzvah; however it is the correct thing to make a meal on the actual day that he becomes bar mitzvah. A meal made on the actual day of the bar mitzvah, isn’t considered merely a party, rather it is considered a “seudat mitzvah”, and is a mitzvah to partake in it.
Yam Shel Shlomo Baba Kama Chap 7-37, Chayei Adam 137-15, Kav Hayashar chap 11.