I treat dissociation in kids and adults abused as children. EFT can help to "calm" and "soothe" himself when he feels overwhelmed, confused, inner conflict (alters in conflict) or when distressed. It's useful to teach SUDS scale of 1-10 rating scale so that he can develop more self-awareness.
It's always helpful to coach him to define a "safe-place" to go to in his Mind's eye . Then once he feels calmer/safe you can help him tap on defined issues, while in his "safe-place". The type of structure really helps kids who feel powerless.
Also, try co-creating some soothing affirmations to tap on when overwhelmed with feelings which for him, escalate into rage quickly. The practice of tapping will begin the cool down process more quickly.
He may not have co-conscousness with inside "parts" but you can begin the process of coaching him to take responsibility for his behavior. This will help him develop co-consciousness ( the beginning of real healing/integration)
Good luck,
SFriedLPC