How do I know he REALLY is interested?

I think you've answered it for yourself!

Flirty emails, going out on dates with you, and a few get togethers means that he is interested. The fact that nothing has progressed could mean that he is shy or afraid of being rejected, or simply doesn't know where your boundaries lie.

I say set yourself up for a get together and leave an opportunity open for him to kiss you.

But ya, he is interested. He's just too shy to make the first move. The ball is in your court!