Aggression, Entitlement, Unpredictability

I have a highly intelligent, freshly-5-year-old. My son understands all of the preteachings (and summarizes back to me) when he is calm. He does SODA exercises effortlessly when he's calm. He is extremely kind, considerate, and helpful when he's calm. He happily does his consequences when he's calm… But he almost always tries to play pretend (to varying degrees), and this creates issues with listening and such. He has hardly any concept of danger or empathy. He is frequently an impulsive bully to others (randomly hitting, pushing, spitting, etc), again. He is a threat to himself and his surroundings when he chooses to lose control; and gets AGGRESSIVE (verbally and physically). Nobody can figure out what triggers the meltdowns, and the frequency and duration vary drastically… So I will use the example of the Calm Down Spot as the basis of my question. When he's out of instructional control, I end up having to wrap my arms and legs around him (firmly, but gently) to get him to stay put, while also protecting myself from his physical attacks. Most of the time, he will fight me for upwards of 20+ minutes, until my husband finally comes in and takes over. And sometimes, even after he has "calmed down," he will turn it into a game of trying to break free, and refuse to stay seated (and still refuse to accept his no answer, consequence, etc). He talks back, including "no, mom, YOU need to accept the no answer" (referencing him telling me that he will not accept his consequences). We have resorted to strapping him into a carseat in his calm down spot, in order to protect everything/everyone, and be able to tend to our baby while my son is choosing to lose control… My husband and I speak calmly and directly. We are consistent. We give him TONS of love, praise, and attention when he's choosing self control. His teacher is on the same page as well (he is a problem-child at school, so we are in frequent contact with his teacher)… He understands it all when he's calm, but his emotions and self-entitlement are wildly out of control (I don't think this can be genetic, necessarily, but I will mention that his biological dad is scarily similar).. We are a calm, stable home. We have strong LDS values. I've prayed, read parenting books, talked to professionals, and nothing has helped much. Your teachings have taken the edge off, though… What are we doing wrong and how can we fix it?