http://thewayofpeace.org/what_is_a_landmark_baptist.html
I am not sure what you want to know, but here is a kind of summary.
They are fundamentalist, and broke off from the Baptists over doctrine and politics, although some of the Landmark churches belong to the Southern Baptist Convention
Each Church however is an independent entity, so it would matter WHICH Landmark Church he is attending as to how cult-like that particular congregation is.
I can get a serious headache wading through all the different Baptist Conventions.