Nothing will happen to your son as a direct result of breaking your vow, as in no magical ill will befall him as a direct result of using that specific time...but the caveat is that he is and will continue to be harmed greatly by your using. I know this first hand, as it became a very real possibility of losing my children due to my drinking. It is said that one cannot get clean for someone else, but your child's well being can certainly be a real impetus for change.
As suggested, read around here and learn techniques for quitting for good. The AVRT threads are excellent. You can do this.