Hi, very short man here. Never been pegged but have had sex with multiple women significantly taller than me. I won't lie, it can be a bit of a challenge if your partner isn't used to being 'taken' by somebody shorter.

If you're trying to go from behind, you're never going to find a comfortable position with your partner kneeling in front of you. You need to be able to position their junk at a comfortable height for you to thrust into. Depending on the high difference, this can be as little as pushing their legs further apart so there junk is closer to the ground, but in significantly different height situations, you may want to get them to essentially lay with something under their belly. Something like a pillow if you're on a bed, or they can use a low table if you're stood up. This way they will be able to support their weight comfortably without having to have their knees in an awkward half-bent position.

Otherwise, just throw him on his back with his legs in the air and gently push on the back of his thighs to give you access. Putting a pillow under the arch of their back can help with this.