Based only on chapters 1 and 2 the supporting evidence is limited.
David is shown as being idealistic right away because he is shown as being a dreamer. Not only is he a dreamer, but he wants to share the mystery and wonder of his dreams with other people; however, that is against societal norms. David's sister tells him to keep his dreams to himself and not tell anybody, but David doesn't understand the point in such a rule.
A second piece of evidence to support his idealism is that he does not turn in Sophie for having six toes. She would be deemed a Deviant and would face the punishment. By the end of chapter 1, David not only agrees to keep Sophie's abnormality a secret, but David also starts to question within himself why a society would even bother having rules against Deviants at all. To David and his idealism, Sophie seems normal enough. He judges her based on his experience with her, not her physical form.
David is too innocent to understand the adult world around him for a simple reason. He is young. He hasn't had enough time to learn exactly how the adult world operates. He still has that little boy mentality of asking "why" about everything. I have a six year old son, and that is all he seems to ask. Why this? Why that? Why? David is old enough to have learned to not ask "why" out loud all of the time, but he is still young enough where "why" is always at the forefront of his thought process. The young age is also why David is so idealistic. He hasn't had much experience with bigotry, exclusion, segregation, etc. He still sees most things as pure and good.