I would only add that we were not ALLOWED to "hold" our M in any way when she was violent, not even to stop her from hitting us (you don't need to tell me its ridiculous) I realize that your son is adopted so, you may no longer be under the watchful eye of an agency/county. But, in our county, we would have needed special written permission and training from our child's therapist to use any sort of hold since we are a "basic" level home (ahem.).
When my 4 yr old fd used to have tantrums, the safest place for her where she would not hurt herself was in her bedroom - I would stand inside the door, not giving her any communication until she had calmed down (I used to stay with her to make sure she didn't harm herself in anyway). During these times she often used to hit/kick me and when she did, I would step outside the room, shutting the door so she could still see me but not touch me until she had calmed down a bit, then would go back into the room. My SW told me she did not agree with me leaving the room but I said I didn't feel it was right to stand there and let her kick/hit me as this gave her a very wrong message - I therefore asked my SW for some other methods I could use since she didn't approve - she couldn't give me any!! I spoke with the child's SW about what I was doing and she didn't bat an eyelid! so I continued with my method and soon the kicking/hitting stopped.