Rear Seat Belt problems of the 2003 Toyota Matrix

I wish to question the safety of seat belt design, as my son has now--3 times--gotten so tangled in his rear-seat belt that it had to be cut to get him loose. We have a Toyota Matrix, but I don't think this problem is specific to that model car. It happened once before in a rental car whose make I don't recall. No question our young, squirmy son did something weird; we think he put the shoulder strap behind him and then turned in some odd way. But after that, the design is such that the lap belt just kept getting tighter and tighter the more he tried to get free. Unfastening the belt did no good. And it seems quite dangerous for there to be no way to release it. There should be some kind of manual override mechanism for such cases. Or at the very least a warning in the car's manual that explains that if a passenger allows the shoulder strap tighten behind the body, there will be no way to loosen the lap belt. The Toyota dealer did replace the belts at no cost. They said they were aware of this happening beofre, and that they'd mention the problem to their design/safety department. But assuming that the seat belt design in most cars is similar, we thought it was a problem to bring to your attention.