I say recessive because, at least in his immediate family, I have not seen anyone else but him and his sister exhibit such a tremendous height. His younger siblings are all average height (the tallest one being about 5'6"-5'7") and his parents/other immediate relatives are either short or average height themselves. If it is the pituitary growth you mentioned it could be a genetic hormonal defect that was carried on by both parents from distant ancestors and neither knew about it. When it comes to genetics it's pretty much hit or miss.
That's not what I was suggesting at all but it's not an impossible theory either..like you said height is polygenic. We are not carrying just our parents' DNA but our ancestors as well. We don't know how tall they might've been. We carry a LOT of genetic information that in most cases is randomly selected and must be juggled/controlled. When genes recombine there are several proteins with a strict code of instructions whose job is to regulate all hormonal processes through life, including height. If a mutation were to occur on the genetic code or the protein responsible for height regulation was absent it's not outrageous to say that, depending on the error, less or more growth hormone would be produced/released resulting in either extreme tallness or stunted growth. Remember we are not born with a set height. Even external factors such as exercise and diet can alter height to a very small degree in some cases.
There are genetic conditions that cause short stature and if I'm not mistaken that also cause giantism. Tumors can also be genetically influenced. Anyhow, I don't remember much of the segment as it was aired some time ago but I think that the cause was in fact the overgrowth gland tumor.