My son, (16M, Asian, 5'8, 177lbs) passed away from Diphenhydramine intoxication (>20ug/mL) any way to estimate how many pills that equates to?

My son passed a little over a year and a half ago, but it still drives me crazy at how it happened. I just keep coming back to wondering how many pills he actually took, I did ask the medical examiner, but she didn't seem to even want to take an educated guess as to how many it was or maybe she sought to spare me the knowledge?

We had generic sleep-aid in 50mg gel caps in the house at the time of his death, but it appeared that only 12 capsules were missing. Some of the missing ones may have been taken by one of us, we aren't sure. Also, he could have gotten some elsewhere, but I searched high and low and found no evidence of any additional bottles in the house, his car, or his clothing.

We don't know the time of ingestion, but it would have had to have been between 11pm and 8am on the day he passed, he approached me and was slurring and talking nonsense then had his first seizure around 11am that morning and I called 911, this was followed by multiple seizures and eventually he was no longer breathing by about 1130am, EMTs started CPR immediately as they were already here in the home, they got him to the hospital but he was ultimately pronounced around 1pm.

I don't even know why I'm so intent on getting an answer to this, just feel like it's something I have to know.