You are so right about sign language! My daughter loves her signing classes. Swebster, try to imagine how frustrating it would be if you couldn't talk but had these great thoughts in your head and no one was able to realize that you are capable of having an opinion. I've worked with physically and mentally challenged people. There is nothing more amazing than watching someone communicate for the first time. I've seen children who were supposed to be mentally challenged and with the help of various types of equipment and therapy, communicate...OMG, what an experience. I met one young person who, with therapy, went from someone who teachers thought was developmentally delayed to reading, writing by using a computer and ended up skipping two grades. The frustration with people who can't speak often comes from just trying to make themselves understood and that would make anyone act out. I really admire anyone who works with developmentally delayed or physically challenged individual and I also know first hand how rewarding it is to be a part of a person's life who has these challenges.
In terms of the meds it would be a good idea since you work with this individual if you could advocate for a second opinion with another Dr. In my job I advocate for services for all my students.
Is this person verbal or non-verbal? If he/she is non-verbal it might be their frustration level is high due tothe inability to communicate their feelings. Even if this person communicates people with mental retardation are limited in their language usage. I work with severely handicapped students daily. The non-verbal students get really frustrated/angry and bite, kick and hit themselves . They also tear and throw things when they are angry. So with that said using functional sign language with this population helps to bring the language out.
Good Luck!!