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Hi - my name is Kirk and I'm a litigation attorney here to assist you.
The only way you can do this legally is to file a court petition to be appointed as his legal guardian/custodian. If you get a court order giving you the right to act on his behalf and in his best interest, you can take care of all of his needs - including making sure that he takes his medicine.
Beause he's an adult, this is the only way that you can take control and give him the care he needs.