Girlfriend's Son has ADHD & ODD

Hello,
I've never posted to a forum before but i need advice. I am a single father of two young daughters ages 7&8. About a year and a half ago my girlfriend moved in with us with her 10 year old son. He suffers from ADHD and ODD. She told me that he had these disorders prior to moving in but i had no idea exactly what the severity was as i have never dealt with mental disorder before. Soon after moving in the boy's defiant behavior started to cause problems. The child refuses to listen to me, challenging me at every turn. He talks back to me, disrespects me in my house, lies constantly, and steals from both me and my daughters. Theres even been times where at dinner he didnt like what was being served so he tasted it and spit it out on the floor. He shows no remorse for his actions, he doesnt seem to even think that he is wrong for his actions. Last week he stole some earrings of mine out of my jewelry box and took them to school to show off. I noticed they were missing days later and asked both him and mom if they had seen them. Both of them said no and that same day the teacher handed me my earring aand said she found him playing with them at recess. He denied they were mine, i spoke to his mom and she said she didnt know anything about this. Finally when mom was at work i questioned the boy again and he admitted that he stole them AND he told his mother and she instructed him to lie to me about it and that she would cover for him. When i called mom and told her he admitted that he took my earrings and that he told her about it, she denied it , said he was lying on her and instructed me to spank him for lying on her?!!?!?!?!?! Who does that??? i told the boy i wasnt going to spank him because i knew his mom was lying and she later admitted that she tried to cover for him but what a mess im in. I cant trust the boy or the mom anymore. No temper tantrums or aggressive or violent behavior just a constant state of defiance and a certain smugness about himself that drives me absolutely crazy. I usually picks all 3 of the kids up from school everyday and his mom gets home from work late nights so i spend alot of time with him...more than his mother during the week. I do not discipline him and his mother doesn't either so he has no consequence for his actions. All he gets is a "TALK" and at worse he cant use his phone for a week. My daughters now cant stand him because of how disrespectful he is to me in my home. His mother has begun taking him to counseling because i threatened to put them out and his father is in jail for the rest of his life on a number of charges. Hes never had his father in the home and Im the first guy his mother has dated and the boy is jealous that his mother is in a relationship so he steals from me, lies to me, and defies me every chance he gets. What was once a peaceful home with me and my girls is now a war zone between the boy and i with mom in the middle. She has a very strong allegiance with her son of course so she defends all of his actions and never gives him the type of punishment that might make him actually think twice about acting up again. I love his mom, my girls like her alot too and shes great to them, but the boy is a nightmare and i dont want my girls to grow up thinking that all of this is normal. Im also concerned about when the kids get older of the possibilities there. I think this boy might do something to my daughters out of spite because of his resentment towards me. Mom downplays EVERYTHING, even though me,the school,his counselor, all his teachers, students, summer camp, etc. all have issues with him she still refuses to change how she parents him. Without a Father this boy has no fear and no respect for authority. He runs all of his mom and im helpless because hes not my child. Im exhausted and im running out of hope. I dont want to spend any more time suffering through this. Please advise!

Are you worried about what they'd do if you broke up with them? Why do you stay together?

What do you think the boy gets when he defies you. Does he seem pleased? What are the emotions he displays?

Is the boy on adhd meds? He might be creating trouble for stimulation.

Oh $#%^, sorry this was from 2016. I did not realize.