I know antipsychotics are often used to quell aggressiveness.
But given that you say he isn't sleeping well, that might be indicative of depression or anxiety (or both).
It sounds like he's on a lot of medications already, and very possibly SSRIs could help him. But medication in moderation is an excellent rule to live by since they can create problems of their own.
Perhaps, if his verbal receptive skills are good (i.e., he is capable of following some verbal instructions), then some form of Behavioral Therapy might be a healthier alternative to adding more medications.
Plus, don't forget, even if his mind isn't as old as it should be, his body's still going through puberty and the whole roller coaster ride that comes with it. Maybe giving him a safe place to physically vent his frustrations might be helpful to him. Provided he learns to mostly keep those frustrations to that particular place.
Does he has any other forms of communicating? Sign language? Etc?