Alhamdulillah, I think my son understands the basics of Tawheed ar-Ruboobiyyah inshaAllah… next is to teach him Tawheed al-Uloohiyyah (singling out Allaah with worship so that none besides Him is worshipped, so that none besides Him is called upon)
“Say: My prayer, my sacrifice, my life and my death are for Allaah, the Lord of the worlds. He has no partner. This is what I have been commanded with and I am the first of those who submit (to him with Islaam and Tawheed)” (al-An’aam 6:162), and He said, “Therefore turn in prayer to your Lord and sacrifice (to Him only)” (al-Kawthar 108:2).
And to teach him about Tawheed al-Asmaa was-Sifaat which is describing Allaah the Most High and naming Him with whatever He described and named Himself with, and with whatever His Messenger (sallAllaahu `alayhi wa sallam) described and named Him with in the authentic narrations (i.e.. the 99 names of Allah subhana wa t’ala).