I am so very sorry to read about your loss. Try to remember that your boy left you while you were there, and he was happy. That is a true gift. No pain, no illness.
I promise I am not saying that out of the blue. I lost my girl Morgaine suddenly, 3 weeks after her sixth birthday and 5 days before Christmas of 2006. I left home that morning with a silly, happy girl who got off the elevator on the wrong floor and made me chase her to get her back and to my own apartment. No clue at all that anything might be wrong with her. By 3 pm my unflappable friend and dog walker called in a panic that she was in massive seizures in the apartment. I rushed home and spoke with my own vet on the way and he told me that is sounded like she was in status epilepticus, and that it was highly unlikely that we would have a good outcome but that he knew I would take her to the evet near me. Nothing could be done. We have no way of knowing when she started seizing. We only know what time Kelly got there.
I am still not completely over beating myself up over it. I know that she always had the best medical care. She had a "mom" who adored her. She had a vet who loved her. She had a walker who loved her and worked so very hard with her to alleviate her SA. She had patients at the nursing home who loved her visits. There was something inside of her that I had no control over. My vet likened it to people who collapse while crossing the street, or athletes who die without warning.
I want to say don't beat yourself up over this, you couldn't have changed it. I know that is true. I know that no one else is thinking anything close to that about you. Clearly, you loved and cared for your pup and are heartbroken.
If it helps at all, I did a lot of searches and put together a whole Excel spreadsheet modeling the possibility of ever going through that experience again (I adopted within 10 days) and found that the results of such a sudden death were ridiculously low. Less than 2%. It is rare.