I am a retired toxicologist and have dealt with his "problem" for almost 50 years.
Almost very community has periodic "take back days" sponsored by the US DEA, in which a pick up is scheduled. You can look up the location and date on google. I believe last week was the designated DEA take back day. Some other communities have regular, permanent sites. If all else fails, take some non-edible substance, such as coffee grounds, crush the pills and mix with that and pack in a contain and discard in the regular trash. in fact dumping in the toilet used to be the recommendation and really still makes most sense, although not politically correct,since it is being frowned upon by the powers that be. When you realize that all medicine taken by an individual leaves their body in their urine or stool, either unchanged or metabolized by the body.
PM me for further discussion.