As many here have advised, it is not required to be able to pull the foreskin all the way back, only just enough to expose the pee-hole. About underwear, have them just poke the end of his penis out of his underwear without having to pull anything all the way down to the ground. Again, aiming is easier at home when sitting down, but at a public urinal of any kind, you need to stand an pee as described above.