I'm dealing with the same thing with a dcb 3 1/2. He constantly tests me, i feel bad because he is always in time out. Parents are not on board and constantly makes excuses for his behaviour. I'm giving my 2 weeks notice tonight. I will literally tell him not to such and such and explain why and he will look at me and do it any way. It's so frustrating. I recently started a family of 3 which 2 are 5 month old twins and I'm honestly exhausted dealing within this boy then with the babies.
Honestly if you can afford it I would terminate or look for a replacement then term, it's not worth the stress and frustration, not to mention it's not fair to the others as it takes time away from them.