If you are as mature as you say, then why are you worrying about what size his FLACCID penis is? You say the sex was good, so I do not think I understand the issue.
Yes, if you are used to a circumsized penis, seeing an uncircumsized one is different. And now you have seen it. So there you are. I would suppose that, when he was an infant, his parents did not believe in circumsizing their child. He is apparently not Jewish. That would be the *reason* his penis is not circumsized. How did that foreskin affect you? You say the sex was good, so I am assuming it did not. You can take this to mean that it is just something that changes the appearance of his penis, while flaccid.
Unless there is a hormonal issue, and you would most likely know about this if there was, there is no reason. Why are your breasts the size they are? Why are his feet the size they are, or yours the way they are? That is the way he is, there is no *why*.
You have been dating for a few weeks. Get to know this guy. If he is as good a guy as you say, then you will not care.
chickita