The fact that you see your brother decompensating and that he has developed a tic may prompt the need for medical attention first. While he may not want to seek psychiatric help, your brother may be open to a physical exam. I think this might be the first step.
Your love and concern for your brother may be something that can help. Find a way to talk to him about the tic as something that should be checked out medically. You might even offer to go for your own check up at the same time with him. Support for him during this time might be very important.
In any case, a physician would be in a good position to make recommendations for a diagnosis, medication, or therapy.