Answer: I would not expect your height difference to be a major problem, but you might try putting a pillow or two under your knees to raise the level of your hips.
What may be the problem is, he is trying to insert his penis in the wrong direction, down instead of up. Remember your vagina angles up towards the small of your back. He should not be pointing his penis straight forward or down.
You might try inserting something of the appropriate size into your vagina, perhaps a dildo if you have one, or a tampon applicator (prior to inserting the actual tampon into your vagina), while you lay on your back, then roll over and get on your hands and knees, while the object still inserted. This will demonstrate that you can do it, and show him the correct angle of insertion.
There is the chance your hymen is till partially intact, and covers the lower portion of your vaginal orifice. When he enters from the front, and points his penis down, his penis slips in over the top of your hymen and into your vagina. When he tries to enter from behind, the hymen blocks entry, as the opening is facing downward, away from him. Examine your vulva with a mirror to see if your hymen could be the preventing entry from behind.