Try a genital massage BEFORE oral sex. Get some oil, similar to Don't Stop by Booty Parlor, or Oil of Love by KamaSutra. Don't use too much, because he will cum right in your hands... but use enough to keep his dick lubricated.
Start massaging around his penis first. Start by sweeping in through this thighs, almost as if you are scratching him, but more of a tickle. This relaxes his penis and his body. Then start by rubbing his pelvis, all around his shaft, but NOT touching his penis yet. Then start gently massaging his scrotum, or lightly scratching/tickling. You can get rough with testicles, they aren't really that sensitive for most men. As long as you aren't racking them with your hands lol.
You can then start massaging the shaft with your hands. He may not be flacid, but this doesn't mean it doesn't feel good. Do not jack him off, only massage it. As he gets harder, take the palm of your hand and place it lightly on the head of his cock. Rotate your hand around it, while barely touching it. This adds stimulation and will increase sensitivity during the blowjob.
Then, the most important thing is to massage the perineum. This is the area underneath the scrotum, and extends to the anus. You will feel a hard bump, and that is were you massage it. Sometimes it wont get hard until after you massage it. You'll know when you've hit it because he will tell you that it feels really good. Use two fingers and massage this as if it was your G-Spot. Using up and down or circular motions, getting rougher and rougher. But don't press it too hard unless he is okay with it. Pay attention to his face.
You can continue this throughout the blowjob by keeping your hands around his cock in a diamond shape, with your thumbs underneath his scrotum massaging the area. Try swirling your tongue around the head of his cock while it is in your mouth. Makes for an amazing sensation.