What should I do about having bought something my son swore I wouldn't buy, knowing that telling him will lead to animosity?

Fulfilling an oath by the one who swore it is religiously encouraged. Obeying one's parent in matters that benefit the parent and do not harm the child is obligatory. If a father swears an oath upon his son not to do something, and the son does it, then the father has broken his oath, and expiation is due from the father who swore the oath. If the father is unaware that he has broken his oath, he must be informed of it so that he can expiate for his oath. It is permissible for the son to undertake the payment of the expiation on his father's behalf, after obtaining his father's permission.