Everyone says don't 'prep' your kid for ERB. But what does that mean, that I shouldn't allow him to play with blocks or puzzles??

That's moronic. Of course play and read and enrich and talk to your child. Look at maps, do puzzles, build towers and talk about the world and how things work. Draw pictures and practice letters and drawing shapes and patterns. Play games with your own kid, duh! You don't have to pay someone $20,000 to do this with special worksheets for some overrated test -- you can and should be enriching your child's life as their parent.