Your have good reason for your sadness. If he is currently acting out and hooking up I'd say you need to take steps to protect yourself. This is a boundary he should not be crossing for so many reasons, a primary one being your own personal health and safety. You deserve so much better. His victimization should not be allowed to trump you and your children's happiness and health.
If I've misunderstood the situation, please forgive me.
Lots of love,
“Life’s journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting ‘Holy ____…! What a ride!’” ~Hunter S. Thompson