One thing you should certainly not do is forcing something. If it's still attached don't force it. It isn't covered and attached the first couple of years for nothing.
I don't know if it helps you but the medical encyclopedia that I consulted stated the following:
?...At birth this skin is normally attached to the glans, but sometime before the third year the skin should be able to be retracted in such a way that the glans is exposed completely.?
I don't know how old your nephew is but the advice of dances12 sounds very sensible.
By #13219 at 28,Oct,10 22:21
If you do retract it MAKE DAMNED SURE IT GOES BACK INTO PLACE. Serious problems if it will not go back into place. If you need to get it back into place, lube with vaseline and then pull the skin over the head. If this does not work after several attempts then to the E R. But DO NOT LEAVE IT RETRACTED
By #13219 at 28,Oct,10 22:24
probably leave it to the mother to ask the doctor on her next visit to the pediatrition
By #32325 at 29,Oct,10 06:34
thanks it just i was not sure as u am circumcised
By #13481 at 28,Oct,10 16:36
when my two boys were young baby's we got told by the nurse to pull the foreskin right back to wash it when in the bath this seems to the right thing to do to me .
?...At birth this skin is normally attached to the glans, but sometime before the third year the skin should be able to be retracted in such a way that the glans is exposed completely.?
I don't know how old your nephew is but the advice of dances12 sounds very sensible.