Hey i'm a guy and i've experienced this too.. The reason being, the "banjo string" ( the string of skin attaching the foreskin to the head of the penis, is pulled back as he inserts pulling the skin back untill it pulls this string which yes does hurt alot........ So me and my fiance tried finding ways around this.
I'm not sure if you mean he can't pull back he skin at all or just back a little.. But heres what we did
1) make sure your moist, get him to play with you for a while, this is like a natural lubricant.
2) try lubricating his penis, or sucking him for a minute so his penis is wet
3) insert his penis slowly untill it hurts then pull out again then repeat eventually his penis ill be wet with your natural lubricant and will slide straight in
if he can't get his skin back at all, he may need to be circumsized
hope this helps you both
good luck!