We ordered the retail version of Lego EV3, and my son loves it and wants to do more.
Are there any add on kits we can buy to expand upon what we have, ie add on motors or special tech pieces, or an educational kit?
There is an expansion set, as well as a selection of additional sensors, etc. you could purchase, listed on the products page of the LEGO site's Educational Section:
Note that these tend to more expensive than the retail kits, and tend to have slightly different items in them.
There have previously been a number of 3rd party suppliers making sensors for the NXT and RIS systems, so I'm sure they will also provide support for EV3.