Wow how scary to watch that!! I would have had a stroke!!
I have 4 boys, and will have a 5th in June... and while on the one hand, my first instinct was to say "Yes... that sounds like typical boisterous, boy behavior"... I also had to stop myself, because at some point you have to draw the line between being a "rough little boy" and being sincerely dangerous, whether it was intentional or not.
I would sit down with Carter when he does come back (with his parents maybe?) and tell him gently how much it scared you, and how much you love your son and that you want him to be safe and not get hurt.... and that you are so glad they can play together again, but he has to play nicely and carefully, and not push *anyone* off of *anything* again.
In our house I have a large white board on the wall with "House Rules" written in permanent marker on it. There are a list of easy-to-read rules for my boys and their friends... maybe you could put something like that up to remind them both how to play carefully but still have fun?
Its also a good reference and detours you from having to be "the evil mom that sent my friends home"... you can just go to the rule board and say, "Okay guys, if you break a house rule you can't come into the house. Remember that." and then if anything happens again, you have a visual aid to explain why you're sending them home.
Remember, the male species is extremely visual! Having face-to-face talks, and using things like the rule board have *drastically* helped me in maintaining a good household!