To be a little more helpful, it might be the case that he needs to be stimulated in a very specific way to come, and that when you blow him you are not doing it it exactly the way he needs in order to climax. Let me tell you what I mean.
My ex-girlfriend would give me long blowjobs and I would not come close to coming from them if I was only passive during them. So I would tell her to let me control the angle and speed and I would do it in the way I knew would make me come, and it always worked. One time she had blown me for a long time (nearly 20 minutes) and wanted to stop, but I really wanted to come in her mouth. So I told her to give me 3 minutes to "fuck her mouth" and come. She grabbed my iphone and went to the timer and said go. I came in about 2 minutes. It was easy.
So if you really like giving head and want him to come from it, maybe let him take complete control of it. I don't mean violent face-fucking that it not pleasurable for you. I mean simply the angle/positioning, speed and depth are under his control. I know that when she was being passive and I was fucking her mouth to come, I would go very slow and rhythmically. It wasn't difficult for her to take at all. Good luck.