I wish I had some parenting advice for you. My 13 year old also has ADHD and with him nothing phases him or matters. He's currently failing multiple classes so we took away his kindle and 3ds. Doesn't make a lick of difference. He just goes and lays in his bed all evening. It kills me because he is really smart too.
This can be a tough one, but you really need to show him who's boss, and let there be a consequence for his bad behavior/actions. Whether this is time out, being grounded, toys taken away etc. I think kids someetimes really try to test our patience and see what they can get away with, but i have been consistent lately in following through with punishing my kids for their bad behavior, or talking back. My daughter has had all her electronics taken away before, twice actually, for 2 weeks, and i tell you, it killed her! She knows now i'm serious and i don't think she will be in a hurry to act up again. It does take a few punishments for them to realise you're serious. The main thing is for you not to lose control of your kids, once that happens, and they know it, it's all over. I've seen it so many times, parents just give up because they just can't control their kids any more. Also, kids know too when you're stressed/busy with other issues, and think you will just shrug most things off because you're too occupied with something else, (my kids were like that when Bailey waas born, thought i was too busy with the baby to notice they were getting away with what they liked). Do you have any male family members he is close to that could maybe help you out also? Sometimes this is what boys need too when they don't have their dad around all the time either. I know this is truly a hard one, hell, I'm going through it too! But remember, we are the parents, and we must stay in control!