back seat belt came undone after being secured

I had seat belt come undone a couple weeks prior to this event. I moved my son from the seat that had the issue with the seat belt into another seat. Today his car seat, which is now anchored in, thanks to a kind car salesman that assisted in showing me where to properly anchor the seat, fell over when I went around a curve. His seat belt had come undone. So this is 2 out of 3 seat belts in my back seat that are not staying secure.

I was almost in an accident and when I slammed on the brakes to avoid hitting a car that pulled out in front of me my son's carseat flew forward. The seat belt had come undone and was tightly secured earlier that morning. Since my son is small he cannot undo the seat belt.