Gallienus proclaimed his elder son Valerian II caesar

During his Danube sojourn (Drinkwater suggests in 255 or 256), he proclaimed his elder son Valerian II caesar and thus official heir to himself and Valerian I; the boy probably joined Gallienus on the campaign at that time, and when Gallienus moved west to the Rhine provinces in 257, he remained behind on the Danube as the personification of Imperial authority.