It would be very hard to know what caused his death without a necropsy.
Can you send the body to your state lab? What state are you in, we can help you look that up.
If the state lab is not an option, performing your own informal examination may shed some light on the cause. Pay particular attention to the liver, heart and other internal organs. Posting photos of what you see can be helpful, someone may be able to help recognize a problem.