Explain to him that this is not a good idea, and try to convince him to keep taking his medication.

Tell him that if he stops taking his medication, he will more than likely need to be hospitalized again

Respect his right to make his own medical decision, since he is currently competent

Tell him he is obviously not in his right mind, since he is thinking about stopping his medication, and that you will have to involuntarily commit him to the hospital if he does not agree to continue taking the medication