How do you prove that your son was under prescribed medication that took his life while he listened to music?

Pharmacy gave warning of dangerous mix of medications that were prescribed by his doctor. All would cause suicidal thoughts and toxic reactions to the medications, anxiety, nervousness, inability to think straight, mixed complicated thoughts and the list goes on. He was in the garage in parents carwhich he normally did not do, he listened to music in his truck that he could not drive into the garage. He was spitting up blood, refused to go to the hospital, and was confused, but listened to the music, simply fell asleep, or as the death certificate stated, he committed suicide and died from carbon monoxide poisoning. His life was lost, he promised his parents that he would not ever do this, he had made plans to marry in January after his divorce, he was extremely happy with his children, and his future new wife. Everything points to the toxic medications that caused complications and would cause suicide thoughts hundredfold. There is a need to tell his story, but how do you prove this?

In a suspicious death like this there is always an autopsy.
When the results of the autospy are made known then and only then could you prove that medications were the cause.
If this was not your family member,it would be hard to fight what the death certificate says and after burial it would be next to impossible to exhume the body to do the tests needed to prove toxic medications.
I am truly sorry for your loss but just because all outward appearance show that your friend was happy,maybe he wasn't as happy as he made everyone think he was.
With your description of the incident and promises he made to his parents I suspect he wasn't as happy as he appeared and there may have been some things going on that he didn't share with others.
If he was as confused as you say he was,why didn't anyone just take him to a hospital? Why was he left to his own devices?
Good Luck in your endeavors but I think you are stuck with the cause of death on the death certificate unless you can convince family to do something else in finding out what happened if other than suicide.

I am so sorry for your loss, or the loss of the parent(s) that you present here. Also his children, his ex, and his future wife. Unfortunately, you can not change what has already been decided by others. He was prescribed these medications, some of which have the same side effects as most antidepressants, for a reason. And the pharmacy informed him of the risks, which he felt okay with because he wanted to take the meds for whatever reason they were prescribed. Again, I understand wanting to get the Truth out, but no two people have the same Truth when it comes right down to it.

Sounds like they were over prescibed