How to discipline my almost 6 year old?

My son will be 6 next week and I've had so many problems with him from his school. From kindergarten and now first grade. The teachers say, he hits other kids, says means things, is constantly interrupting, rarely does what he is told, and doesn't want to follow directions. I've got a note from the school at least twice a week now...and they are thinking about expelling him from school! I've tried many different punishments and so has the teachers. Here is my problem...I am a very strict parent, but his father isn't. We are not together and haven't been for at least 5 years now. I've tried to take him to court but cannot prove the courts that he is not a suitable parent. He is an alcoholic, smokes weed, is training in mixed martial arts so thinks he is a bad *** and starts fights with everyone for no reason. He is always spending all his money on himself and the little young gf's he meets from the school that he helps at. When my son is with him on the weekends he see's all this. His father let's him play video games pretty much all day. He play fights with our son but actually hurts him! His dad and his grandparents from his dads side pretty much let him do whatever he wants. I have him Monday through Friday...and his Dad sometimes pics him up on Friday after school and I get him on Sunday evening. How can I discipline my child if the other parent is teaching him bad things and doesn't discipline him? My son just doesn't seem to really listen to anyone else except for me. He can be pretty bad when he is with me...but not as bad when he is with others. Punishments- I have put my son in the corner, sometimes with a book to hold straight out. I have taken away his play station...taken away some of his favorite toys, made him write his alphabets over and over again and much more. The teachers have taken his recess time away most of the time or would make him sit in a chair next to them ...and nothing seems to phase him at all. It's really worrying me...I honestly don't know what to do anymore and do not want them to kick him out of that school. If any of you have suggestions I would be very happy to hear them. Thank you so much