There's this boy on our school coach's AAU team (he doesn't go to my school) and I see him around because they have practice after us most of the time and I just got officially introduced to him yesterday.

We didn't have a real conversation. It was just "J this is my friend Leah." "Hi Leah." "Hi." and then we saw eachother later that day and I just said "Bye." So I feel like if I do acknowledge him he may have forgotten my name or he might think I'm creepy or something.

Not just guys, but anyone: am I allowed to say hello next time I see him or should I wait until I've had a real conversation with him? Does it matter?

Of course you can say hi to him. It shouldn't matter because he knows who you are.