What you think may have been a suicide, may have indeed been an accident. Many drugs can interact in strange ways and cause death when taken the same way as before, and the person was okay. This even happens with street drugs. Lost my nephew when he used a drug he'd been abusing for 10+ years and his body just couldn't take even one more "regular" dose.
Sometimes bad things happen but usually, without a note, is not a suicide.
Just give him comfort but it is not appropriate to ask if it was a suicide or talk about your suspicions. He will get a report in time.
And again, it will probably be ruled an accidental death as most are. Leave it at that. It's tragic enough.