S.H.
Hi M.,
I think your worries are racing way ahead of your son's actions! He's written a note. He is "attracted" to little girls his age. This is not sexual. It is natural, normal and OK.
As for dating, you just explain the rules when he gets old enough to care. He is into play dates right now. Next year he'll probably have had enough experience with the ladies to know that they aren't as fun as boys and before you know it, they'll have cooties.
He probably recognizes caring and kindness and calmness in little girls, which is comforting because those things remind him of you. He's away from you all day. He's likely just looking for the things he feels at home.
Don't worry about dating yet. Don't call attention to what he is doing, either. Ask him why _____ makes him feel good. Start a dialogue with him where he can share his thoughts with you and not feel shamed or misunderstood. His ability to talk to you (or feel judged and not talk to you) will carry into those scarey teen years.
When the time comes, you make the rule about dating starting in high school. And you lay down the law and stick with it. If you're consistant with your rules he won't fight too much. Remember that you will still be the adult. He might have a "girlfriend" in school before then, but they'll be in school and those things are more something to talk and gossip about than a real relationship. But let all that go for now and enjoy the innocense of a 7 year old boy.