I think that it is wonderful that your child has developed a love of reading!  In order to foster that love of reading, you need to make sure that when he is reading independently, he is choosing "just right books."  Just like Goldilocks and the Three Bears - not too hard, and not too soft (easy).  When your child is reading in small group time with his teacher, his teacher should be using text on his instructional reading level. This text will be a little challenging for him and will give his teacher data to use to plan the most appropriate instruction for him.  When you said that "he pretty frequently" comes across words that he doesn't know, that raised a red flag to me that perhaps the books he is choosing are more instructional level and not independent level.  A quick rule of thumb is the five finger rule (not scientific by any means).  Have your son to hold up a finger every time he comes to a word that he doesn't know.  If he gets to 5 pretty quickly (especially within a couple of pages), then that book is probably a little too challenging for him.  This would probably be a book that you should read aloud to him, modeling fluent reading.  You can help him to choose a similar type of book with similar plots, themes, and characters, but on a lower level.  The whole goal of reading is to comprehend - to learn new things or simply for enjoyment.  If children cannot read the words enough to comprehend, then they reach frustration level.  Too long at this level will cause children to hate to read.