Noncommittal : Not expressing or revealing commitment to a definite opinion or course of action.
He did not go through with his course of action in buying the watch.
Flighty: Fickle and irresponsible.
He was irresponsible for wasting money.
Capricious: Given to sudden and unaccountable changes of mood or behaviour.
Changing his mind about the watch was an unaccountable change of behavior.
Whimsical could also work. Similar meaning to capricious.