I don't see any ich cysts either. It's possible he could have died from a secondary infection.
One thing you should understand about ich is that ich bugs very commonly exist in almost everyone's aquarium, but since it is so prevalent, most tank-bred fish like bettas have built up a resistance to them. Because of this resistance, fish only actually become infested with ich if they are under some kind of stress that weakens their immune system--such as low temperatures, temperature fluctuations, poor water quality, rapid changes in water conditions, and other stressors. You should carefully evaluate how your fish's living conditions may have contributed to his infestation. Purchasing a reliable liquid master test kit and a couple of glass in-tank thermometers is a great idea, if you haven't got them already.
Sorry for your loss, best of luck with your next fish.