Have you talked to his case worker/social worker/treatment team about these concerns? They are going to be a better resource than reddit, I'm afraid.
There are certification classes you can take on therapeutic restraint and other tactics for handling rages and meltdowns - consider checking those out.
Is there a crisis team you could call? Check to see if your area, agency, county, etc. has support groups or other resources for foster families. There are often church-connected or nonprofit ones as well. It may feel like you are alone, but you are not!
Consider reading "Building the Bonds of Attachment" by David Hughes (https://www.amazon.com/Building-Bonds-Attachment-Awakening-Troubled/dp/0765704048/ref=sr_1_2?ie=UTF8&qid=1496171891&sr=8-2&keywords=building+the+bonds+of+attachment). It covers a really similar situation - a child who has violent and destructive rages as a result of trauma, and discusses therapeutic interventions (including holding). It looks like a really dry, dense, inaccessible book from the cover and title but it is actually really readable and really useful. Work by Karyn Purvis (http://empoweredtoconnect.org/) also has a lot of strategies for managing and healing this kind of behavior.
And, as always, document, document, document!