Not to worry you are not alone.  At this age reading can be tough for kids and they can become frustrated with it.  The best thing that you can do is keep encouraging your son to sound words out.  It sounds like he knows some words but not all so have him use what he knows to help him with the words that he struggles with.  If this doesn't work then try to make word games out of it.  Making flashcards of all the words that he knows may help as a reminder when you go through them.  Don't be afraid to help him out and sound it out for him and give him the confidence that he needs to reassure him that it is ok not to know everything.  Their is a great computer website that can help him learn to read and it is www.starfall.com try that.  It will review letters and sounds and word parts to make certain sounds and it even gives them short stories that he can read and hear and the same time so he can learn to recognize the words that he may be having trouble with.  Most of all stay positive and try not to get frustrated because when he sees you get frustrated he will become frustrated as well and don't be afraid to take a break when things become intense.  Just walk away from it for a few minutes and then come back it will give you both a chance to cool down and then come back with a more positive mind.  Hope this helps.