Your two year old is most likely doing what two year olds do: copying Mommy. That's it. He's trying his best to nurture as you have nurtured him. Kids this age get the general picture but get the details wrong and you can see how he's doing just that! Lots of little boys this age try to nurse their stuffed animals and other things/people. They stop doing it after a short while. It is, in my opinion, very unlikely that anything perverse or sexual going on with your son. If I were you, I'd show him that nursing happens from the "bo bo" part of the body (the chest) and it doesn't happen from the penis. For the sake of social propriety more than anything else, I would tell him NOT to nurse toys etc from the penis by just saying something like "No, not that way; we do it like this (and then show him how to "nurse" properly)." I wouldn't even bother telling him that little boys don't nurse since this behavior will disappear on its own very soon and I'm sure he won't "get it" when you try to discourage him right now. He's really far too young to understand anything about this. He's just playing copy cat and not doing such a great job of it.
In short, please don't worry about it. Your kid sounds normal and bright!