out of control 4(almost 5) year old... help

My sons completely out of control, he won't listen, throws himself to the ground and screams when he doesn't get his way... He's almost five and I'm 29 days away from giving birth, I'm tired and my patience is thinning... Today I spanked him for the first time and it still didn't work:( not to mention I cried after spanking him because I felt like crap... I have tried timeouts, warnings and he's on a schedule so he knows where his day is going... Idk what to do, I feel like I'm failing as a mother and my husband thinks I'm to soft with him, that I should whoop him but I just can't hit him ... I just don't know what else to do! He has been evaluated and has a sensory issue but even his OT person says his behaviors are a result of personality not his sensory issue... Help ladies ... Because I can't keep wrestling my 50lb child in public, chasing him around and dealing with his disrespectful mouth (he tells me no, laughing) ...