I’m sure by now you are aware that it wasn’t your fault, even if calling 911 right away would have saved him. And that there is a very good chance it wouldn’t have saved him.
You were 14. You reacted like a 14 year old. Did the best you could. I hope you’ve found a way to know deep in your bones that you acted with compassion and the strength to keep yourself together, and that’s pretty damned admirable, even if the decisions you made were not the ones you would have made at a different age.
And being there with him at the end means… ummm… well, I was there for my daughter’s first and (25 years later) last breaths, and I think it means something. Your uncle met the end accompanied by somebody who cared about him. Maybe that logically doesn’t mean anything, and I don’t understand why it would, but most of us bald monkeys over the last few tens of thousands of years think it’s important.