I've recently started dating someone that I met online.
There are no rules about exclusivity. It is whatever you and your dating partner are comfortable with. Of course there are extremes to be avoided. You probably don't want to be exclusive after 1 date or wait for 50 dates, but thats obvious. In my experience it was always less than 10. I think from what I have heard from other people, it is usually less then 10 for them as well. I think where you are at right now, is a perfectly good place and time to bring up the topic.
I guess its natural to start to feel some jealousy toward the fact that he is dating others. However thats just part of the territory of dating. I would think you guys will be making a decision on exclusivity soon, so your fears in that area will be put to rest hopefully shortly. I always found it a weird topic, the other people you are dating when you are a date wiht someone else. It was never brought up in my experiences. I think I even lied about what I was doing on a certain night when I was on date with another person. I just thought there was no harm in lying about that. Even though it would have been perfectly fine to say I was on a date, and a very honest answer, I just figured lets enjoy our date without thinking about others.
Don't be confused about why he is on the site as much as he is. He is still dating others and not exclusive to you yet. Keeping his options open at this point, which is always a good thing before you commit to someone. Don't be guarded, its perfectly ok and does not raise any flags for me. Also men are lookers. I think they tend to look a lot more then women.
His joking about your going out with the girls and about the messages you are receiving, tells me that the idea of you being with another man sorta bothers him too, so you are both becoming attached and i think you two will be an item soon.