You could look into ReadWorks.org or RazKids. A lot of people here don't like RazKids (and you'd probably have to pay for a subscription), but my 7 yo DS likes it. I'm not sure if there is an assessment, but there are books that they can read on their own or listen to the audio, then multiple choice test questions and they can earn points to buy virtual prizes. Readworks has comprehensension questions but doesn't automatically score them like RazKids. It's more something that you'd print out and give him like a reading passage and then a worksheet to go with it. Readworks is free.
