I am so sorry about the sudden loss of your dog. The fact that he was so young makes it harder even to deal with. My heartfelt condolences.
The only way to have really known what happened would have been to do the autopsy. But of course, you are right, that won't bring him back, and you probably preferred to leave his body in peace.
Sometimes a blood clot can migrate from a surgery site. This isn't common, but can sometimes happen. That would have been no-one's fault. If so, it could have gone to his heart or brain, causing a sudden fatal stroke. He would have passed quickly, and not known a lot about it.
There may have been another cause....I can't say for sure obviously.
Bless his Soul. I am very sorry.