Practice makes perfect, so keep practicing. Try to find books that the child finds interesting. For some books, you can read a page, and have them read a page, which will help them keep interested.
You could ask the teacher if there are particular sounds or patters your son cannot sound out, and if there are books or words you can read which focus on the problem areas. For example, a short U sound.
Phonic awareness seems to be one of the most necessary but overlooked study in school. It promotes great reading, spelling and writing skills and yet the elementary schools do not spend enough time covering the basic phonics. It is recognized as the leading reason why high school students struggle with reading and yet it is still overlooked in the elementary grades, where it should be covered.
Unfortunately, we can't change the school curriculum, but we can teach these skills at home. I think it's great that you are involved!
I've provided some links below that will provide you with resources and free worksheets and http://www.starfall.com is a great site that is child friendly so your child can work alone, it's fun and helpful. Best wishes!