Sister’s jackass boyfriend just told my kids Santa isn’t real

I don’t usually post on here but I need to vent I’m so fricking livid. We have the family together at my parents’ for Christmas. It’s my parents, me and my husband, our 2 kids and my sister and her boyfriend. I didn’t really like the boyfriend before, he’s kind of a scrawny Reddit nerd stereotype. He has said some suspect things in the past but we have kind of shrugged it off. But tonight, he took it too far.

We were watching Rudolph, and when it ended, we told the kids they need to go get in bed to make sure Santa comes and brings presents. This motherfricker proceeds to stand up, stretch, look right at the kids and say “Santa’s not real. He’s a fake guy. Your mom and dad already brought the presents.”

Immediately, the 6 year old starts tearing up, asking us if Santa is fake, while the 3 year old bursts into a tears, crying and screaming. My sister looks mortified while the boyfriend just stands there smiling with this smug, self-satisfied look on his face. My husband looked like he wanted to kill him and he started toward him, but I put a hand on his chest and told him to go outside and that it’s my family so let me handle it. He’s still in the backyard, my parents are trying to calm the kids, and my sister cried and went to hide in her room after I lost it on her about the boyfriend. I’m posting this, about to go up to help with the kids, and the boyfriend is sitting on the couch watching Netflix without a care in the world.

OT, what would you suggest in this situation?