WakeWorld - Looking for a BMX bike for my 5-year old

My son is almost 5 and has been racing a dirt track three days a week for several months now. To say the least, he's obsessed. Since he is obviously still growing, I'd rather spend my money on a used bike that another child has outgrown than on a brand new bike. I am okay with dings and scratches. The max I will spend is $350. The bike will be primarily used for dirt jumping and dirt racing, so no front brakes are preferred. I think a 16 is too small, and I've heard that an 18 or mini 20 are good bets. Brands we dig are Redline, PK Ripper, Fit, and GT. We are open to other suggestions.

Are there any forums similar to WakeWorld, but specifically for BMX, where I can get more info about the type of bike we should be looking for and the best place to find a used bike that meets our specs? I've already posted on Craig's List.

Thanks in advance!
Anne-Michelle