The Kindle Fire might be a good way to go... Personally, ii like my Nexus7 a little better, and that's just personal preference. And that's where this is going to boil down to. Some people are going to tell you what to get because it works for them... But that's just it: Them.
One reason I picked the N7 was the freedom it gave me. I can even load Ubuntu on this thing (why I would choose that crap OS, I have no idea) with a single app download and some screen taps.
If your son is technologically inclined, and he likes to tinker, don't abandon the N7 so quickly. But, if he's just going to use it to play a couple of games and check E-mail (like you said) and has no desire to play with the system, then you might want to look to save a couple of dollars.
The problem is, at $199, it's hard to save much by avoiding the N7.