Without knowing the age, I will make several suggestions. Hypnotherapy can be very effective in bringing relaxation and allievating anger.
Polarity also brings relaxation and whatever lies behind the anger could be brought forth and eliminated. Although not a well known healing modality, it has been around a long while, is very non-invasive and can be very effective.
There are also herbs that assist the nervous system to relax. Chamomile and oats are two common ones that come to my mind.
Lavendar essential oil can also be very effective. Can put this on a cotton ball near the bed at night or rub a little on the soles of the feet. Rescue Remedy from Bach is also a good choice. Taseteless, can be added to water or put directly in mouth.
EFT is simple and effective, too.
Diet is also important. Eliminating dyes, sugar and possible allergens can be very effective, too. Read Doris Rapp's book, "Is This Your Child?"