Sherri, I am sure this is frustrating. If your son is still having trouble distinguishing vowel sounds and the teacher's only suggestion is to spend more time reading together, then I would probably seek another professional opinion from someone who can actually see your son read -- maybe another teacher, a tutor, someone from administration, etc.? It may be that he is just picking things up late, but if there are any underlying issues you want to be sure you pick them up *now*, and all the time in the world isn't going to help if there is something that is creating the challenge for him.
Until you can get that organized, I can think of a couple of things that might make reading a little less frustrating. First, since you think he is having trouble with vowel sounds I would make sure he knows the general rules for determining whether a vowel sound should be short or long. Most words in easier books will follow these rules, so it will cut down on the number that are difficult to figure out. Second, can you take turns reading? Maybe he reads one page then you read one page, or something like that? Both when my children were learning to read and with the elementary kids I tutor in reading (a volunteer thing -- I am not a certified teacher ) it seems that the kids do get tired really easily and that short break each page (or every few pages really seems to make a difference. Finally, there is a publisher (I think it is Scholastic) that makes a line of books specifically designed (in terms of subject matter and interest level) for older children who are reading at a beginning level. One of the things that may start to be frustrating for your son is that the books he is able to read are too young for him, and the books he is interested in are too difficult. He might be more willing to read something that he is interested in. For that matter, I would say just about any reading material is probably fair game at this point - comic books, etc. all require the same reading skills as traditional books.
It doesn't matter what you think, just that you do.