5 year old very mean to dad, I feel like dad is responsible. I'm at a loss.

I'll try to keep this as short as possible! My 5 year old son is very mean to my SO (his dad) and is what some call a "momma's boy". We had him at a young age and my SO spent a lot of time out when he was a baby so for the 1st year it was mostly my son and I at home. After a bit of a rough start, SO has gotten his shit together, and really provides for us and has fully been involved in his life.Now this is my current issue I could really use advice with: Our son has become very mean to his dad. It started with him getting very jealous when my SO would show me any affection (this still happens) The it turned into our son not wanting any thing to do with his dad at all. Now he wont hug him, and says very hurtful things. He will say things like, " I dont like you dad" & will get excited when his dad leaves to work. Its just constant nastiness towards his dad when Im around. It even has gotten physical a couple of times, where my son slapped his dad.Now here is where I feel lost: My SO is a very angry man. He has a short temper and reacts out of anger very often with both me and our son. It has caused 90% of our disagreements and fights. When our son acts mean towards him his first reaction is to yell or spank him. He yells very loudly which causes our son to cry, then he yells at him to stop crying, or he will get a spanking. wash, rinse, repeat. I know this is not the right approach! I will try to step in, but he will get upset with me and say I spoil our son and that I dont respect his parenting. My SO works swing and graveyard, so he is tired often. On one occasion he slapped my 5 year old on his face for saying he hates his dad. To be honest, I reacted like a normal mother would react seeing someone hit her child, and I went to protect my son and comfort him, which made my SO extremely upset with me, and he refused to talk to me for almost 2 days.My son is not a bad kid. Other than the usual 5 yo naughtiness and exploring boundaries I dont have any issues with him and his teacher at school tells us he is a pleasure to have in class. Even though I also work, I make an effort to spend a lot of time with him going to the park, events, and at home crafting and cooking. I know my SO is really exhausted because of his crazy work schedule, but on his days off he doesnt put a lot of effort into bonding with our son. They used to do jiu-jitsu together which seemed like it helped, but now my SO will just go to class and take my son to tag along and watch him train.I feel so distraught, Im caught in between protecting my son and respecting my SO. My SO comes from a really awesome family. His parents are just genuinely nice people who really just love their kids, so its not like my SO had a bad example. For reference, I have a daughter from a previous relationship who he treats very well, but maybe because shes only with is part time? I feel like he is very depressed with his life, personally hurt, and maybe disappointed with our son. I feel like deep down he doesnt love our son like he expected to. I want to fix this. I would like for my son to treat his dad better, and I would like my SO to be nicer. Any advice would be amazing. via /r/Parenting http://ift.tt/2f8oXsz