The answer is probably: touch him.
Erotic desire is about touch: his chest, his mouth, his penis, using your hands, your mouth, and your penis.
What we call "sex" is actually about touch: fondling him, kissing him, putting his penis into you, putting your penis into him.
You can enter his mouth, but then you won't be able to fondle him anymore. You can enter his butt, but then you won't be able to kiss him anymore. But guess what: he has a third place for you to enter while still being able to fondle and kiss:
Between the legs.
It's called intercrural (Latin for "between the legs") or interfemoral (from the femur bone that extends down the thighs).
Your partner lies down, face up, and squeezes his legs together. You lube your cock, lie on top of him, and enter just beneath his scrotum, the area called the perineum.
While you're thrusting, you can kiss and fondle as much as you like.
Interfemoral allows you to feel his entire body, not just his mouth or his penis.
I generally like to start the erotic session with kissing, fondling, and oral. When the guy finishes, I enter for interfemoral (the perineum is particularly sensitive after an orgasm).
If you're big or husky, he might find the pressure uncomfortable. In that case, you can enter in an arched position.
If there's a major size difference, I suggest putting your partner on top.
You can also turn him onto his stomach and enter that way, as an alternative to anal.
Interfemoral is completely safe. The only problem might be a friction burn on your penis, which can be avoided by using lube.
You and your partner can't do interfemoral simultaneously, but the next best thing is called frottage or the Princeton Rub (because Princeton students used it as an alternative to anal):
You press your cocks together and manipulate them with your hands, or use the pressure of your bodies.
Frottage works best when the cocks are the same size. If one is substantially smaller, it gets lost.
Interfemoral doesn't work with three people. I've tried entering between his legs while the third guy enters his mouth, and entering from the front while the third guy enters from the rear, but it's a complex operation with little erotic payoff.
Interfemoral requires you to focus all of your time, concentration, and erotic energy on one person, on exploring his body as fully as possible.
Isn't that what sex is all about?
See also: The Ins and Outs of Oral Sex; The Most Underrated Sexual Act; How Intimate is Your Sex Life?