B.H.
I don't think your son's behavior is uncommon. I've heard about behavior and have seen behavior like that alot at that age. If that makes you feel any better.
My daughter is in kindergarten and comes home daily with reports of this little boy in her class kissing all the girls. My daughter is terrified of boys and things it's just gross but apparently some of the girls in her class like the attention and do it back. I think at this age it's harmless curiousity and they're just really curious. Acting out what maybe mom and dad do at home. I would definately have a talk and keep reminding him of appropriate touching and playing. I wouldn't worry about it unless it continues at a older age.
My daughter had a playdate over last week. A little boy. Being a only child she was extrememly excited. She chased him around and flirted and kept pretending to tackle him. Then he asked my 5 year old to be his girlfriend. I thought it was really cute and funny. Until my little girl was hysterical and was hiding. She kept saying she was scared and she didn't want to have boyfriend they all have cooties.