Ugh! Scary stuff! One of my nephews and one of my stepsons experimented (and were caught by me) w/each other when they were 4 and 5 and my stomach was sick about it for days!!
I would be inclined to chalk it up to normal experimenting, because kids do go thru this stage....HOWEVER, the cousins sound too old to still be in this "experimenting stage"...absolutely keep it in the back of your mind when/if the cousins get together again...always watch them, like a hawk!! Hopefully the parents of these boys will take this seriously and take the appropriate measures to ensure they will not be doing this to anyone else...ever again!
I agree with the other mom that said leave your son alone about it for awhile...seeing how he thinks it was just play, and wasn't traumatized by it, don't push your fears on him, he will probably forget it ever happened!
Then in a couple weeks start talking to him about "good touch/bad touch" and to always tell mommy or daddy if ANYONE wants to touch him or have him touch them.
It'll be OK mom...he will probably forget the whole thing ever happened, I mean it!