Is one obligated to fulfill a promise if it is highly probable that fulfilling it would lead to harm?

You must repent to Allah for breaking a promise to your child. Fulfillment of the promise is not obligatory if it leads the son into disobedience; rather, fulfilling it becomes forbidden. You should appease your child and compensate him with something that benefits him and encourages him to study, while being keen on connecting him with righteous company.