general problems of the 2010 Subaru Outback

My sons (7yrs old) was sitting on passenger side rear seat with booster seat and seat belt on. When we got home, I shut off the engine and he un-buckle the seat belt and was playing in the car. Next thing I know the car (un-buckle) seat belt wrap him around. (I can forward photo of how the seat belt wrap him around(like a knot). . He was unable to free himself. I tried and was unable to pull the seat belt loose. I even let the safety(seat) belt "relax" but still unable to pull extra length of belt from the "c" pillar to loosen the "knot" around my son. . . . . As he tried to free himself by "twisting his body around, the seat belt tighten whenever the "knot" around his body gets smaller. . . . I took me almost 3-5 minutes to free him from this "know" by pushing the knot down pass his leg (like someone taking off their pants). . . . . I felt something is not right with this seat belt as there were no way for me to pull extra length of seatbelt to loosen the "knot". As you can see, if any adult not around, a child may get panic and try to free himself/herself by sliding down (ie. . Like taking off a t-shirt) that this belt design that automatically tighten (and does not loosen when relaxed) would post a choking hazard, please let me know how I can upload/send the photo of the "knot" to you?.