I don't question how he goes about it, whether with books or "screen time" (welcome to 2013, where that is becoming a fuzzy, fuzzy line.) He downloaded around a dozen math apps this past weekend alone. Last night he was watching a video and doing a practice question on Pythagoras Theorem and understanding it. I would be a fool to interfere with this.
I've been wrestling with this issue myself--if DD9 is *reading* something, but on a computer/Kindle/iPad, and it's educational, should that really count as screen time (i.e., in the sense of something that should be limited)? Actually for written stuff she prefers books or magazines and I had a hard time getting her to use the Kindle until she realized we can get books on it *right away* sometimes instead of having to schedule a trip to the bookstore, so she's slowly changing her mind. To say nothing of, she's wise to the fact that DH and I use computers all the time, all day, for work--so to be fair, isn't it a little weird to say she should only look at a screen for a much smaller amount of time a day, regardless of content?
Sorry--as to the OP, I too would be offended, but I would let it go for now just because I think it would be more advantageous to you and your DS to simply work some kind of comment into a later conversation along the lines you indicated and how much your DS has learned, in a positive way. But I have been burned by trying to call people on stuff where I thought they were clearly way off base and having them get all passive-aggressive on me, and getting nowhere after that. But clearly it works for some people, and you are a better judge of that...just keep in mind that as hard as it is, calling people on stuff may not be what's best for getting your kid what they need and you want. That may make it easier to bite your tongue for the time being
Good luck!
