Masego's very good at detecting and defending against mental influence, but he does have a crack in the wall of certainty he'd otherwise throw up. At the point of canon he's at, he's recently learned some things that suggest his parents are hiding information from him, which implies through certain canon metaphysics that his incubus dad may not actually view him as part of the family and could just be playing the doting papa for his human father's benefit. (Short version: all devils are deterministic creatures, and incubi have the core expression of "satisfying desire". Masego greatly desired the information Tikoloshe could have told him, and the contract that wed him to his husband allowed him to answer the desire of anyone in their family. Therefore, if he failed to respond to that desire, he must not see Masego as part of the family.

In fact the explanation ends up being that Tikoloshe, alone among every devil since the beginning of time, has gained actual, real free will through the influence of his husband and son, and withheld the information out of the desire to protect his child from something very dangerous. oops.)

do note that masego is not likely to fall for this for long, and he will be epically furious once he snaps out of it. which I for one encourage.