I'm responsible for his death
What did you do to be responsible for his death?
I was paying too much attention to social media when he drowned in the river
It is good you acknowledge where you may have went wrong.  I am sorry for your loss in this situation, and can only hope that you can learn from this grave mistake.
When I was watching him, he tripped and I didn't catch him.
I understand how upset you are about it. But now, nothing can be done and it is in the past. You must move on from this.