My 14 year old brother still gets skid marks on his underwear? he doesnt have a medical condition? this normal?

Yes it can't always be avoided. Sometimes tiny fragments of poo may remain even after u have thoroughly wiped all your holes clean and dry. Tiny droplets of pee or l secretions and discharges may escape. And if u unexpectedly expel liquid instead of gas when u fart...! Hopefully all this will be trapped in your underwear and nowhere else!