Sorry for your loss, but there’s no way you could know that. The very same logic you would require you to espouse the belief that any person who received a vaccine and died, died because the vaccine.
He was in mid 30s, young for COVID - but diabetic. There have been actual studies done for the vaccine to show efficacy in preventing death for someone in his risk group. So not the same logic, but accrual proven causation. There's a very very good chance he would've made it had he been vaccinated.
Sorry for your loss, but there’s no way you could know that. The very same logic you would require you to espouse the belief that any person who received a vaccine and died, died because the vaccine.