Not that I have any experience on the giving end - but you might try incorporating the use of hand manipulation along with the oral activity. The not knowing of the next touch whether it is from lips, tongue, finger tips usually drives a guy wild. As the moment of no return arrives, then go back to strictly oral and you'll both be satisfied. Good luck and let us know how your efforts turn out. He's a very lucky guy by the way for you to seek out advice!