For such a small tyke I would consider the weight of the bike and the size of the bike to be really important. Keep in mind that 99% of the bikes you see in bike stores and are 200 bucks plus are designed by guys 5'10"ish and weigh about 160+ pounds.
Gears are not part of bmx and you really won't find that in a 20" bmx bike, though there are some 'small' mountain bikes you may want to ask about if gears are really important. Gears require a lot more maintenance and can cause you headaches if you don't want to keep them up, don't know how, or don't want to pay a bike shop to do it once or twice a year.
Now, the Free Agent Maverick is one of the smaller racing bikes and should be no problem supporting your son. It's only $150.00 from www.danscomp.com
and should last at least 3-5 years with proper maintenance.
That bike should work until you son is about 90-100 pounds pretty darn reliably. It is not a toys-r-us bike, but is not anything near what many bigger riders would look for specifically because it is smaller and lighter. Lighter ALWAYS translates to weaker unless you are paying a ton of money for premium parts (good titanium/steel/aluminum etc.). I would be willing to give up some strength for the several pounds you save on that bike. It weighs in at 26 pounds, while most of the bikes you will see in shops and around danscomp will weigh in between 32 and 35 pounds(ish). You can get pegs for it if you want, but your son will likely just ride his friends around on them until he is actually big enough to put them to some real use. It may just be a waste of money to get pegs and it weighs the bike down.
If you do add pegs, the demolition dumchuck pegs are a minimal weight added peg that should survive the (lack of) abuse that you son can dish out to them.
I think that is what I would recommend at least.
Keep in mind, in a few years it may be time for another new bike, perhaps a $350.00ish 33 pound freestyle bike - but for now, that much money, strength, and weight is overkill and won't be manageable for his size.