"我被他当做(了)自己的儿子。" 干爹 gāndiē or 教父 jiàofù?
It is common to say "godfather" when referring to an elder man who is influential to someone's life and treated as the person's father, but in a non-Christian country like China, simply translating "godfather" to 教父(literally "religious father") is quite improper, if the person ("他" in the sentence) is not from a Christian country or is not a Christian believer. If this is the situation in the context, the elder person ("我") can only be the younger person's ("他") "干爹" instead of "教父".