Matsui said:
It won't be fun since they aren't 21. I'd look at something else.
One of the most fun times I had was at March Madness in Vegas under 21. Most likely they'll never say anything as no one said anything to us as long as he doesn't try to drink or gamble. They're a little busy that day.
Most of the hotel / casino lounges are also converted to sports watching locations. Hooters used to be a blast as well. More like a sports book than local restaurants with everyone there to watch the games.
Any of these would be awesome places to make reservations for while your there. Hell, you could rotate through multiples. I'm sure there's others as well as the ESPN sports bar and the Hooters were both places we enjoyed, but are probably long gone by now.
https://www.thrillist.com/drink/las-vegas/best-sports-bars-las-vegas