Dave,
Reading this post gave me the chills.
When I was 13, I was asked to babysit for 2 kids, one was 3 and one was 7. A parent of a friend of the 7 yo dropped off two more 7 yo kids into my charge. I didn't know better to say no because it was a "parent".
Long story short, the 3 YO put his finger in the door crack and one of the 7 YO's slammed the door. The tip of his finger came off and there was blood everywhere. I went into a sheer panic. I called 911 and was hyperventilating. I didn't know what to do with the kid as he was screaming his head off. I did cover his finger eventually to try and stop the bleeding, but I do admit that at first I had him at the sink washing it off (horrible, I know. I didn't know what to do). I was able to find the tip of his finger (it was so tiny, and severed right under the nail) and I put it on ice. The ambulance came and took him away. I was in severe shock as i'm sure the little boy was.
A few days later, I was allowed to visit him (he was sleeping, heavily medicated, and had his finger in a sling).
Still to this day I have nightmares about this and I DON'T ALLOW anyone to play with doors when there are kids around nor do I allow any kids to play or slam doors. My DS knows he's not allowed to play with doors under any circumstances. This moment changed my life, for the worse. THe family moved away about a year later and this was now 20 years ago. I still think that there is some 23 year old around with a reattached finger and I blame myself even though I know the kids were just playing and I was put in charge of children I shouldn't have been. It haunts me all the time.
Prayers to you and your little boy. Please keep us updated.