I'm so glad you are getting some respite. It is so important!
Have you called his psychiatrist to let him know how he is behaving? I know one summer I was calling difficult child's psychiatrist almost every other day because he was so off. They actually appreciated and encouraged the calls.
Just a curiosity, and maybe this has been covered in another thread, I noticed he is not on a mood stabilizer, often with mood disorders they are needed, has your difficult child been on one?