A few points as I have quite a lot of experience (2 boys, one 13 and one 9!).
I wouldn't get anything too cheap - feel the weight of the cheap bikes! The REALLY cheap ones weight a ton (not joking). Also - have a good look at the brakes on a cheap bike. The ones I looked at were absolutely dreadful and flexed all over the place. I wouldn't risk my kid on one.
Personally, I think Wethepeople are a bit faddy. My son's friends have them and for the same money they have not been as well built as the Mongoose we bought my eldest (Mongoose Chamber - lovely bike. I'd actually like one too!).
Our youngest has a Haro with the 18" wheels. Good solid bike and it's done both of them now with no damage.
There are 3 sizes of BMX. If you go for the smaller models he'll grow out of it quite fast but it will fit him better. The Mongoose Micron might be a good compromise as it has full sized BMX wheels (giving a good choice of tyres) and a smaller frame.
You will possibly have a running battle with him about the brakes

My eldest is desperate to go brakeless, I've been insistant that he should have two brakes. I noticed last week that he's removed the front one. I might have to let it lie and compromise on one brake because I'd rather have him riding than give up - his fitness level is fantastic since he got into cycling and swimming. Peer pressure is a terrible thing.
I'm a HUGE fan of BMXs for kids - they're totally the simplest, strongest bike you can get and they love them!
It certainly looks like it's been a way to lead my kids into a life of cycling.
Edit: if you can stretch to a cro-mo frame, I would. The weight difference is huge.
With my kids the logos were stripped off in minutes because for them overall brand is less important than component brand.