My rough guess (I did not become polyglot until much later, and my second language also had latin script) is that as long as he's having fun he'll be fine. 子不學

My rough guess (I did not become polyglot until much later, and my second language also had latin script) is that as long as he's having fun he'll be fine. 子不學非所宜。幼不學老何為。 (Some friends of mine tried to make diglossia easier by having each language always associated with the same parent. So even if they were speaking to each other, if in front of their son, each would converse in their L1)