E.P.
Some boys are just more fascinated with their privates than others. Hopefully it is a phase. You can tell him to leave his jewels alone but the more attention you give him - the more he may like the attention too. However, you will need to nip this behavior of grabbing and touching others and be very clear and firm that you do not accept that kind of behavior because "no one should ever touch your privates and you shouldn't touch anyone else's privates". Anything that is covered up by a bathing suit is considered "private".
It's one thing if this is happening in your home. However, you don't EVER want to get any phone calls from other parents saying that your son grabbed their child's privates. And make sure any playroom door stays open or semi-open and you peek-in often.
Good luck.