Most commentators identify John Milton as an Arian for his subordination. Milton's Son of God is created but is also the co-creator or demiurge of our creation. The Father creates through the Son, but the Son his the first creature. The Son is divine-like but substantially distinct from the Father. Milton's Christology likely receives a boost on account of the poetic genre since poetic language typically magnifies attributes and actions of characters.
But if you really want to blow a hole in Milton's theology, set your sights on the absence of the Holy Spirit. John Milton's Holy Spirit either plays no role in creation, preservation, or redemption or the third Person of the Trinity has excused himself from the action. Milton seems to identify the Holy Spirit with the poetic muse Urania (the Muse of Astronomy) whom he invokes three times (Bks I, VII, IX). However, Urania is feminine and functions as a "holy spirit" primarily because she is "inspirational."
Just as St. Basil the Great discovered that the Achilles' Heal of the Arian party was its underdeveloped pneumatology, so here, Milton's faulty theology falls into focus. Arianism and other subordinationist theologies simply don't have a place for the Holy Spirit.