It hurts my feelings when my son calls his wife’s father “Dad”. I’m his “Dad”, I raised him and worked hard to support my family, and I never called my father-in-law “Dad”.
Queenie, that name was just for my father who raised me.—Annoyed father
Many people who feel close to their in-laws call them by the same names as they call their own parents. Be glad your son has such a good relationship with his father-in-law. However, because it bothers you so much, ask your son if he could call his father-in-law by another name – something like “Papa Smith”, for example – at least when you are all together, so it is clear which one of you he is speaking or referring to.