My son has started to masturbate

Erika

I tried talking to him about masturbating being completely normal but he had to do it alone.

Well I’ve since caught him twice doing it in the living room while myself and his sister are awake (not in the same room but still risky)

How can I correct this behavior (of him not making it a private thing)

I don’t want to shame him. I just need to protect my daughter. She is 5 years younger than him and doesn’t need to accidentally see that.

Help?!