Mid/rear Seats Assembly problems of the 2009 GMC Acadia

This is in regards to the center row, middle seat belt. My 9 year old was sitting in this seat using the seat belt. I was driving down a hill at roughly 40 mph when the light at the bottom changed so I stopped. My son was leaning a little forward and the seat belt locked up. I told him to lean back and it should unlock. But it did not, it kept clicking like when using a seat belt for a car seat. At which point he became stuck up against the seat and couldn't move his chest. He started panicking so I had to pull over to assist him. I unbuckled the seat belt and let it go in and it locked, completely inside the seat. After pushing in what little slack there was I got it to release and was able to pull it out, but only about 6 inches and then it locked again. I then had to move my child to another seat while on the side of a busy road. The seat belt has remained locked in this position for about 3 weeks. I took the car in last week for an oil change and the recall on the airbags so I asked if they could look at this too. They misquoted us on the price and then proceeded to tell me the only way to fix it was to take the whole seat apart and it would cost roughly $400. 00 and insisted there would be no way for this to be for a recall, as they didn't see it as a safety concern. I have to adamantly disagree. A seat belt locking and not locking could be a very dangerous thing in an accident, or even in a sudden stop.