I'm a dad as well and I have 3 boys all of which went through masturbation states where they became very active with their sexual desires and masturbation. It is NORMAL for boys to masturbate or play with their body in sexual way. There is nothing wrong with your boy nor is there any reason at all that you should do anything or really say anything about it to him.
I would suggest you provide your son with a private room with a lock on the door where he can enjoy himself in privacy without you bothering him. Allow him to fully explore his sexuality and enjoy his own body in any way he feels necessary and as a dad, just stay out of it. He will be fine as long as you don't cause a problem.
With all my sons, I was fully aware that they masturbated but I sure as hell didn't try to talk to them about it. Just let your boy's be boys and STAY OUT OF IT. Let them enjoy their masturbation stage and they will grow out of it and into adulthood as strong healthy men.