If I were in your shoes, I'd probably just schedule a conference with the teacher and generally discuss what he's doing concentrating on the areas that are flagged in the report card. I like to think about what skills I want to see developed and ask questions about how they are working on them / what should be done at home as a general framework. That usually gives an opportunity to talk about your impression of current mastery. I'm not sure what you define as problematic though. Here we only get developing/meeting/exceeding expectations type categories in elementary for various skills including behavioral ones.
That all said, from your description your son has already more than mastered the skills for the year, is happy and you're not seeking additional enrichment. So since the grades in K are meaningless, I wouldn't worry much about it and treat this like a clarifying exercise.
Good Luck
