I am so very sorry to hear of what hapened to your dog. I can't see, from what you said, anything you could have done better. You couldn't have done more for him. Even if you had rushed him to a vet within a split-second I doubt if you could have saved him.
I have no idea what might have caused his death, so quickly and suddenly like this with no signs previously of anything being wrong.
The only things which come to mind are heart attack, or stroke or some embolism perhaps. Though it is sad that had to happen to such a young fit looking dog. But things like that can happen, though they are rare.
My deepest condolences, and I am very sorry for your loss. Bless his lovely Soul.