Just like one is not allowed to call their parents by their first names, which is a Din in Shulchon Oruch, it is a Halacha that one is supposed to respect their father-in-law as well, and one is not supposed to call their father-in-law by his first name. Even when referring about the father-in-law to other people, one shouldn’t use his first name. In a case where there is a need, such as in business conversations, or a similar circumstance, and the father-in-law is clearly moichel, forgives, and allows the person to use his first name, and it is not in his presence but when discussing him to another person such as business associate, there is room for leniency.