When my son was about 6 or 7, he too had trouble with his R’s. I wrote a short story with many R’s in it - more R’s than E’s in fact. I asked him to read it our loud, and by the time he got to the end, there was a small but noticable improvement, which got even better as he reread it in the following weeks. Take a look, and see if he would enjoy Irving, The Dirty Turtle.