Hello, I have gone through your query and understand your concern. There is a significant interaction between the drugs he is taking and haloperidol. Particularly his antihypertensives medications like Metaprolol and other medications like sertraline. I think you have not tell about all history like Hypertension and all other things. These drugs may increase or decrease the effectiveness of haloperidol. You should discuss this with Your doctor and he may change the doses of your medications or monitor you carefully for side effects. Hope this helps. Please write back for further queries. Wishing you good health.