The cheaper bikes at places like WalMart often have plastic bushings in the pedals rather than bearings and are often put together in a rather crappy way.
This is one place where 'steel is real'. Steel tubes take more of a beating than aluminum. Kids treat things differently that adults and a $3000 road bike.
Let him pick out his own helmet. You want him to wear it so it's way easier if you let him pick it... Parents are not known for chosing 'cool' things for kids.
... alternatively, you could just take you son down the bike shop and let him pick one out. Kids have weird logic - you might spend $400 on some cool looking bike and all he wanted was the $120 bike with lots of chrome and cool looking stickers.
For a 7 year old, I would buy the least expensive bike I could find ... and, that probably means one from WalMart/Target/wherever because
I think you have to treat the bike as a "disposable" item which may only be ridden for one or two years ...
AND, a BMX bike really doesn't have many moving parts to consider -- the headset & BB will have been ajusted at the factory (good-or-bad) ...
What the OP may want to do, regardless of where he chooses to buy the bike (but, particularly if he opts for the Wally World bike) is to lube the brake cable with a thin layer of VASELINE.
And, when the "plastic" parts actually wear out, then I would replace them.
Maintaining chain adjustment? Well, that's easy is enough for the end user to do.
Air in the tires? That's up to the end user.
The hubs? Heck if that's an issue, buy another set of hubs (or, entire wheels), later
, OR learn how to re-grease the bearings ... the kid probably weighs less than 60 lbs, and probably won't weigh much more than 70 lbs when he is 10, so the wear-and-tear on the bearings in the hubs & pedals will probably be low ... very low.
I don't know how much a small child can trash a frame ... and, if he does, then the OP will know that his kid doesn't know how to take care of his 'stuff' ... at least, not yet.
There is a huge difference between a 7 year old & a 9-to-10 year old ... so, when the OP's son is 9-to-10, then a better bike from a shop may be in order -- by then, he may want a MTB ... or, even a ROAD bike!
BTW. If anyone wants to see how much you can spend on BMX frames and components, check DANSCOMP.COM ...