Ya i would also say money. For a Bar Mitzvah i would say 75 is the minimum. But since its a good freind of yours 175-250 would be the normal range. Im not jewish but i belive that a bar Mitzvah is a very big event for the boy and its quite an expence for the family as well.
If you give them an amount of money make sure it is a multiple of 18 as 18 is life in hebrew and promotes life. I personally Think that you should give $54 or more. Being Bar-Mitzvah'd I recieved everything from $18 and above. Everygift is greatly appreciated.