People in this country are murdered for pocket change. The dead kid was a potential (and in the eyes of the law,a "presumed") heir to what might prove somewhere down the road to be a 100 million dollar estate.
Given this,I wouldn't bet a single dime that there's no foul play involved here.
His explanation about the combination of drugs sounds ridiculous. It's similar to saying that multiple gunshot wounds suggest an accidental death. There's just so much about this whole mess that screams murder.