Yes, I have heard of amantadine, but not necessarily used to stabilize mood or rages.
Typically a mood stabilizer medication is needed for that - like Lamictal, lithium, tegretol, etc...
My daughter is on a mood stabilizer and an anti-psychotic med (seroquel). It is not unusual for a child to need both a mood stabilizer medication and an anti-psychotic med to even our their mood and get the rages under control.
Lamictal is typically most effective at a therapeutic range around 200mg to 250 mgs - what was your son's dose?
Well, I can understand how this is painful for you to witness...... and its a difficult situation at best.
Is there any chance your daughter would take a parenting class?
It sounds like your granddaughters time with you is very special, maybe you could have her more often?
I know that school and school work will get more and more stressful each year (socially and academically). If your daughter hasn't looked into a 504 or an IEP yet, she may want to start exploring it, as they take awhile to implement.
They need to talk to someone at the School District.
If you pass along your City and State of residence, I can do some online research for you, but your sister (or brother) would then need to call a few people and find out the right environment for your nephew.
There is help out there, its just tough to find it sometimes!