I assumed he was choked by his collar.
I found it a little odd that they said he showed no signs of depression. Now, they don't say WHY the grandparents are guardians, but in most cases that I've seen, it's usually due to abuse, death of the parents or abandonment. In these situations, most kids are going to have some emotional issues surrounding it. Kids don't always show depression in the same way that adults would.
Very sad story.
Dee