Radak: "Ha'Rosh" applies both to Ladan (he was Gershom's Bechor) and to Yechi'el, the Bechor of Livni, i.e. Ladan. Yechi'el is Yachas mentioned above (6:5). Zesam and Yo'el were also Bnei Ladan - there were three in all. Below (26:22) it says that Zesam and Yo'el were Bnei Yechi'el - "Bnei Yechi'el Zesam v'Yo'el Achiv." This Sefer often lists grandsons with sons. Alternatively, Yechi'el named sons like his brothers' names. Or, a Vov is omitted (it should say Bnei Yechi'el v'Zesam v'Yo'el Achiv).