Back Him Up
As he's getting undressed, lie on the bed with your head where your feet usually go, rather than perched on the pillows. As he walks toward the bed, slide down so your head is hanging over the bed, grab his buttock cheeks and take his penis into your mouth. Your head should be off the bed as you give him oral sex upside down. He's ultimately in control of the thrusting, but you can use your hands as a guide to speed him up or slow him down. Not only does this look sexy, but it also opens your throat wide so you can take in more of him without gagging.
The greatest compliment you can give him is looking like you want to be down in that area. And one of the best ways to demonstrate this is to explore all of it. Take one or both testicles in your mouth, hum lightly, suck gently and swirl your tongue around them. (Note: If you don't want to swallow when he ejaculates, switching to testicle stimulation while continuing to stimulate him with your hand is a good alternative.) Also, lick his perineum (between his testicles and anus), and while you're down there...
Give Him "Analingus"
Oral anal stimulation that involves licking, flicking or inserting a stiff tongue into the anal passage and thrusting feels great (for both sexes, actually), because the area is highly sensitive and loaded with nerve endings. If you're worried about germs or STDs, or if you're generally squeamish, put a barrier between the area and you, such as a piece of plastic wrap or a cut-open condom. Lots of guys love this, but many are too shy to tell you. So pretending it's your favorite thing to do (even if it isn't) takes care of that without awkwardness: If you like doing it, well, he has to indulge you, doesn't he?