He absolutely can say anything he wants. But he also has to understand there are consequences that come with some of those statements.
Don't get into a rules clinic with him, he just wants to vent. And of course he knows more, because he watches real football on TV!
This is one of those times you just let it go, taking the high road and staying above it. If it gets too bad, a simple, "That's enough, Coach" is all the warning he should get. If he stops, fine, mission accomplished. If he doesn't, drop the flag. But understand that dropping the flag might escalate the situation. I'm not saying it isn't earned, it isn't the best best option or anything like that. But decide if this is where you want to draw the line. Once decided, stick to it.