Rear Seat Belt problems of the 2001 Subaru Legacy

My 7 yr old son, seated and shoulder-strapped into his booster seat in the back seat next to the window, pulled the middle rear shoulder strap out to where the locking mechanism became engaged, then wrapped the strap around his neck. As the strap went back in it tightened around his neck to the point where he could not get it off over his head or pull it back out again, since it was in locking mode. We were driving in the dark, and he did not tell us he was stuck. We were on a long road trip to relatives and had been driving for about 16 hours, and thought he was asleep in the back seat. Even turning around to look at him we didn't know he was trapped because he had a blanket over him. When he finally started to whimper a little we realized what had happened, pulled over, tried to remove the strap from the lap section of the belt but couldn't figure out how, and ended up cutting the strap to get him out. He was shaken and had some red marks on his neck, but otherwise was unhurt. After our trip we had the shoulder strap replaced.