Should your 5-year old son really be playing a game that says "Mature 17+" on the box, or in my case "16"? These exist for a reason:
If he's running off of the map, costing your team points, the fact that he's 5 shouldn't be an excuse because the game wasn't made for his age group. Now, I will admit that I was playing 16-rated games when I was younger but the thing is I didn't even have Internet at that age. Rules are strict when it comes to online gaming, and it is not made for anyone below the age of 8. If you let him play and you get banned, that's nobody's problem except your own.
I can understand wanting to play Halo with a 5-year old son as I personally don't believe Halo warrants a Mature 17+ age rating but online is not suitable for younger audiences. I suggest your son to play Campaign or Firefight.
Oh and just so you know, we're not the official 343 Industries, we're a community forum.
Their official website is http://halowaypoint.com