An idea for your son to pick up more words when he reads would be for both of you to take the habit to have your 'special little time together' each night snuggled in bed: either each of you take turns to read a page, or each one of you take one or two characters... This will allow you to pick up the difficult words and verify his good understanding... Also, from time to time, you could him ask him questions on the text, especially on inferences...
I spend many years doing this with my DS and in fact, I am missing it so much that I have been volunteering for several months now to take care of a year 4 child who used to have some problem with his reading. Of course, we are not snugged in bed, we are just comfortably installed in his parents' living room, but we are having our lovely time reading together (I love it!!!)
After the summer during which I could read with him often and then just a weekly session since back to school last September, he has moved up during the first term from bottom group to two groups higher up in the class (in English)
This shows how reading good quality books help a child to progress!
(his former 'reading diet' was to read just Horrid Henry books!