If it's at all possible, you should connect the Xbox via a wired connection (ethernet), not wifi. Your son will have a much better gaming experience without the latency of a wifi connection. For tips to improve your wifi connection see: https://www.xfinity.com/support/articles/improve-your-wireless-home-network
My son is constantly getting knocked off the wifi while playing his Xbox.
I have the new modem, and never had this issue with the old one.
Is there something I can do do it stops happening?
I feel you on the matter. I take it that you have the new gateway that looks like a cylinder (XB6) I believe.....if so it is lousy for gaming both wifi and hard wire,though if you hardwire your son's xbox it will be better than the wifi but still, to much latency, either way from my experience. I was getting kicked out of games a lot when I was online, PS4 and Xbox One S. So I purchased my on modem and use my own router now with no issues what so ever.