The P-70 is a weighted keyboard to better simulate an acoustic piano. I have not tried the YPG 525. No surprised that GC hasn't heard of it. They don't always carry the full line. What is light to one person may be heavy for your son. However, if he is OK with the Casio he would probably be OK with most of the Casio and Yamaha keyboard as long as they do not have weighted keys. The lightest touch keyboards I have tried so far has been some of the m-audio keyboards. The ProKeys 88SX is in your price range. It doesn't have the bells and whistles of the casio-types and requires an external amplifier.

So many people start with Casio-type keyboards these days that you may not find piano teachers in your area to be as adverse to them as you think. If the keyboard gets your son enthused about learning it's OK to start with one.

You may want to check out Costco/Sams/BJ's in your area as they often have one-generation back models of these types of keyboards at good discounts.

However, again I recommend that you have your son try anything first to ensure the touch is light enough for him.