Im not a doctor but I could tell you a few things. First I would say why not have intercourse instead of oral? I have found that guys seem much more pleased with intercourse than oral most of the time. I have been told I am the best they had ever experienced when it came to oral. There are a lot of factors to consider when doing oral. It should be personal not just something to do because you guys get bored. It should also be very intimate even try to make it erotic. The simple up and down motion it not exactly sexy so to speak. Try to work in ways to be in more contact like have your bodies closer together. If you were caressing him with your hands at the same time it would also probably be more enjoyable for him. Unless he likes you to just do the same thing over and over try to mix it up often.

I would be more than happy to help further, just let me know.