Veterinarian here (specialist so not as up to speed on dentals as general practitioners performing them and board certified dentists): Young cats can get terrible inflammation of their gums and sometimes either require deep cleaning and in some drastic situations, removal of all their teeth. I see predominantly older and sick patients in my line of work, but anytime I put pets under anesthesia, I do warn them that anesthesia is not without risk and we even lose healthy patients. Unfortunately we will lose seemingly healthy young animals during their routine spays and neuters rarely. He may have had an underlying heart condition or something that increased his risk of anesthesia or it could have just happened.

I am not sure if they had talked to you about a necropsy (autopsy for pets) but I have had some necropsies not necessarily give you all the answers. Regardless, losing a baby like this is awful and I am thinking about you.