I’m very glad you took the time to write. This little boy is highly distressed. The fact that he does well at school and with sitters suggests that the problem is with his relationships with his family members, not a mental illness.
Although you’ve tried many types of discipline, I’m afraid that may be part of the problem. Inconsistent methods of discipline confuse kids. They don’t know what the rules really are. Often they then push the limits until the adults have finally had it. By then, the child has gone way too far and the adults end up doing and saying things they regret. Then things tend to simmer down for awhile — until the next blow up.
There is more going on here than I can address in a letter. Ideally, you and both of the boys’ parents should go together to a family therapist/parent educator. Yes, together. I hope the three of you can put the child’s needs ahead of whatever tensions may exist among you. Someone needs to take a look at the whole picture to see what is driving this kid to such extremes when he is at home. Often things the adults in a child’s life think are not all that important loom large for a child. By talking to all three parental figures, a family therapist may be able to tease out the dynamics that are reinforcing the bad behavior.
The fact that this little boy does well in other situations is a very, very hopeful sign but it won’t last forever. Please don’t wait another week or month to get the help you need to address his problem — both for his sake and for everyone else’s.
I wish you well.