If you're going to buy new, go with the Mongoose Mode 900. Here's why:
-full chromoly frame & fork
-chromoly handlebar
-chromoly 3 piece crank
-double wall rims
-sealed cassette

There's a seller on eBay blowing them out at $169.99, or they can be had from Walmart for around $200. For the price, they can't be beat. Any full chromoly frame set will be $500+ from other bmx company's, and chances are they'll have single wall rims.

What to look for in a bmx is a full chromoly frame, fork, & handlebars. It's stronger & lighter than mild steel. There's a lot of mash ups between the 2 in bmx bikes. 3 piece cranks are lighter, & you have more flexibility for upgrades over a 1 piece crank. Double wall rims are far stronger than a single wall rim. Ideally you want a sealed rear & front hub, bottom bracket, & headset for low maintenance. Of course, if 1 is just starting out then none of that is dogma, but it will save money in the long run.

Any bike you look for in your price range will be mild steel, with maybe 1 chromoly tube in the frame, mild steel handlebar, single wall rims, & 1 piece crank.

If your sons interest drops from bmx you would have no problems selling the Mode 900, & without much of a depreciation loss.