6 year old is out of control

How do you discipline a 6 year old my husband and I have a very hard time disciplining our 6 year old he does not listen at all he acts like he doesn’t hear us when we ask something he hits his little brother all the time we have tried everything rewards for better behavior charts taking away toys taking his tablet time outs not bringing him trick or treating to show we weren’t messing around and this kid does not care at all he continues to do things he shouldn’t be doing he even acts this way at school and they are just as stumped as we are they don’t understand why he acts the way he does either we just recently had a meeting with his teacher and she said she won’t be surprised if he gets popped in the face by another student for the way he acts. Last year his teacher thought he could be on the spectrum this years teacher isn’t sure about that and I have no clue what the heck is going on with him he thinks his behavior is ok he definitely pushes my buttons and it’s honestly hard to connect to him at all because of the way he acts it’s so frustrating I have two other kids 2 and 1 and also a baby girl on the way I’m 24 weeks pregnant I’ve had so many pains and Braxton Hicks contractions that get kind of painful at times because I feel like I’m constantly yelling at my oldest this is and everyday thing I feel like I’m gonna end up having this baby early I had my 3rd at 33 weeks now I keep stressing myself out trying to deal with my kid that’s being a devil child and I def don’t need this baby coming early at all.