Make an appointment to discuss your son's reading with his teacher. Talk about how frustrating he finds it to read aloud with you. Ask the teacher if he or she has noticed similar problems in the classroom. Also, find out if there are any informal reading tests that the teacher could do to pinpoint any areas that need to be improved, or if further testing is indicated.
Reading time with children should never be a frustrating ordeal -- it just kills their desire to read. You need to lighten things up. Forget about his reading to you for a while. Instead, read to him every night. Not only will it increase his interest in reading, it will also give him a better idea of how to read with expression. And more importantly, it will rekindle an interest in reading.
Besides reading to your son, find some easy materials and read them aloud together for 10 minutes each evening. Have him sit slightly in front of you so you are reading into his right ear. As you read, either he or you can slide a finger under each word as it is read. As he begins to find it easier to read aloud, choose more difficult materials. Within a couple of months, you should notice considerable improvement in his reading. Plus, your reading time will now be enjoyable.