How To Handle Other Daycare Kids

Okay this my first post so bear w/ me. My son is almost two and is already in his fourth daycare. The first two I moved and the third one we had to leave after the director told me to whoop my son. I just settle into a new job and he just started at his daycare. His first few days he loved it and wouldnt pay any attention to me leaving. The perk of why I chose this daycare is because I could watch the cameras on my phone because I’m a helicopter mom as much as I try not to be. One day I was anxious and decided to look at the camera and I saw this big two year old kid push my son and smack him in the face repeatedly. Keep in mind my son hasn’t ever seen physical violence let alone any form of violence in his life, he doesn’t even see when his dad and I argue, so when it happened he just froze and sat there and looked scared but he didn’t cry from what I could see. My heart literally just dropped to my stomach because he was literally being beat up and wasn’t reacting to it, not that I would expect him to know how but still. The teacher didn’t see cause she was handing another kid from Across the room, which working with kids myself i understand how fast things can happen, however I called the daycare immediately and told the director to grab my child from that room. I tried to calm down after that but couldn’t so I went to pick him up early. I did speak with the director and teacher about the incident but have yet to let it go since considering the fact that my son has never been in a situation to where he would have to activate his fight or flight mode and I have no idea if he would even comprehend it if I teach him how. Ever since then every time I drop off he cries but when I watch the camera the little boy doesn’t seem to be near him that I’ve seen but does seem aggressive with toys and other kids as well. I’m willing to talk to the other parent about this and the school but considering I don’t them I don’t know how the conversation is gonna go with the mom and I don’t know their environment, I just want to help my son tackle his fears while having to avoid fro switching daycares again because Ik hitting kids is bound to happen at any daycare, but also I want to avoid that this doesn’t happen to my son again. Advice?