Yes, you absolutely want to try putting it in

You absolutely do want to at least feel his pussy against your micropenis, even if it turns out that you can't properly penetrate him.
"Please," you respond, "I want to feel your pussy on my cock."
"Okay, let's try it kind of diagonally. I've had trans guys fuck me with their bottom growth this way before. This shouldn't be too different," he says, spreading his legs as you shift so that one of his legs is between your thighs. The term "scissoring" comes to mind. You push back the fat around your dick with your hand, trying to make your cock stick out as much as possible. It's wet with precum, twitching between your fingers in anticipation. You line up your cock with his entrance as best you can and and push forward. The hot wetness of his juices on feels amazing on your cock, and you let out a little moan.
"Am I in you?" you ask, embarrassed that you can't really tell.
"Kind of! Let me just..." he shifts his hips, and reaches his hand down, finding your cock and holding it in place as he raises his pussy to take it in. The combination of his pussy enveloping your tiny shaft combined with the friction as he moves makes you moan louder. My cock is in him, you think, I'm not a virgin anymore.

For a moment, you and John both remain still, the air between you filled with your own shaky breath. You want to fuck him, to make him cum with your cock, but honestly right now it's taking everything you have not to cum. You move your hips a little, trying to thrust into him, but you feel your cock slip out of his hole.
"Fuck," you hiss. You try to push yourself back into him without reaching down, but your little dick just slides against his labia. Humiliation washes over you. Your fears are confirmed - you really are too small to have sex. John wraps his arms around your lower back, helping to keep your cock against him as he grinds his pussy on your cock, and right away it's too much for you.
"Shit!" you gasp, feeling your cock throb. You groan as you release, flooding the lips of his pussy with cum. You stay pressed against him, panting as you cum.

"That was so fast! I guess it felt pretty good, huh?" John asks, laughing a little. If your face wasn't already so flushed, you'd turn red right now.
"Yeah," you pant. Finally, you pull away from him and lie on the bed. Physically, it had felt good. Amazing, even. But...
"Are you okay?" John asks, looking at you.
"I'm fine, it's just..." you sigh, "I've always worried that my dick was too small to properly fuck someone, and now I know it for sure."
"That's not necessarily true. I mean, doing what we were doing, I would've cum in a few minutes if we'd kept going. Your situation doesn't mean you can't properly satisfy someone," John assures you.
"It's not that," you say, shaking your head, "It's not being big enough to stay inside. I just wanna be able to thrust into you. Not being able to move without slipping out made me feel so..." You trail off, looking for the word.
"Small?" John offers.
"I guess. It just felt so wrong. Maybe because in all my fantasies, I have a bigger dick," you tell him, covering up with a blanket, "All in all, I think I preferred grinding. At least that's more or less the same regardless of size."
"Yeah, I get that," John says, resting his head on your chest as you drift off to sleep.