Hi Kizudo,
From what I understand this is quite a normal progression...there is even a very short reference to it in the YBCR handbook, that includes this as a step to see that your child ( as babies) learn how to orientate the words...Dr. Titzer does mention that you do not want to focus on this too much, as it is of 'minimal benefit'...
That being said, his advice is definitely geared more toward babies just learning to read! Alex will be three in December, and she routinely does this...I think as long as they are reading well, you should run with it...better for them to be reading and enjoying it than disliking it because they are forced to sit still!
And fluent readers, adults included, all do this. It happens every day, and I think it is just a part of fluency...I don't think I would encourage reading upside-down, but if it happens as he is bouncing about, go with it! If it looks like he is guessing at a word, slow it down and have him sound it out. He will probably need to look at it more analytically and in a regular fashion for this, and you can be reassured that you aren't missing anything.
If he reads fluently upside-down, can sound out words/decode upside down (as well as right-side up
), I would just be thrilled that his fluency has reached that level. It may well be a fun game for him! Maybe he will be an inorganic chemist, where rotations, reflections, etc., are necessary for classifying molecules