I do not understand how someone can refuse a higher dosage of a medication that is helping but not helping enough. Would he refuse to increase the dose of a heart medication if he could get relief from a higher dosage? Would he suffer with an eyeglass prescription that is years out of date if he could change his prescription and see 20/20 again? Anti-depressants are kind of hit and miss anyway - some work for some people and not for others. Some get almost immediate relief and some take weeks to build up the medication in their bloodstream. Doctors know that over time, a person can become sort of immune to an anti-depressant and need a bump-up in dosage or a change to another medication. Maybe your relative doesn't need MORE of his medication but less - he could be having more of an anxious response which can make someone irritable.
He should go and talk to his doctor. Is there anyone he will listen to, whom he trusts and respects? That person could say to him, "Listen, you are not acting like yourself, we're seeing real cruelty and you are hurting family members. I am concerned, will you talk to your doctor and tell him this?" It has to be said nicely, and out of concern for HIM, not for your feelings. You could let him know that it's possible he's getting too MUCH medication or needs another kind. If the doctor is still evaluating him for a response, then he is in the trial stage and nothing is fixed yet.
This is hard. I know depression affects the entire family, not just the depressed person. I can spread misery far and wide when I am in one of my down cycles. Pray for him, try to ignore his comments, but also don't coddle him - if he's nasty, tell him he's well out of order.