Am I obligated to fulfill my vow to buy my son a motorcycle, even though he has grown up and it is no longer suitable for him?

The basic principle is the obligation to fulfill a vow if it is an act of obedience (qurbah), based on the Almighty's saying: "And let them fulfill their vows," and His saying: "They fulfill vows." As for vowing something other than an act of obedience, such as permissible matters, nothing is obligatory according to the more preponderant opinion of scholars. Some scholars have gone to the extent of giving the vower the choice between fulfilling it or expiating for it with an oath's expiation (kaffarat yameen). Therefore, the questioner is not obligated to anything regarding this vow, though if he offers an oath's expiation for it, it would be more cautious.