Do you tell your partner you don't like their kids or just pretend forever?

So, my SO has 1 child (8m) with a nightmare of a baby mom. I nachoed that situation a couple months ago and have felt so much better about it all. Now my issue is that whenever my partner gets his son for visits he basically REQUIRES that I participate in whatever it is they want to do.

I run my business from home so I have tons of free time that's just for me and what I enjoy so whenever I talk about wanting his days off to not include me he gets all sorts of upset because I "have enough time by myself and he wants to be a family on those days"

Fair enough. But I'm tired of pretending. I hate it. I don't want to. I literally tense up and cringe whenever he's around. I attribute a lot of why he's the way he is to a lack of parenting and guidance on both parents parts. He's spoiled rotten, incompetent, lies all the time, manipulates to get his way, is rude, messy, and borderline brain dead from playing video games every free second he has.

I cannot stand this child. I have tried. I put on a happy face. I pretend. But I'm tired of being fake af and I don't think telling my partner that I don't like his kid is going to go over well.

I wonder if anybody has been in a similar situation and if eventually you started to like the kid? They grew on you? You could tolerate them? Cuz I don't know what to do other than speak honestly with my partner about this and maybe suggest he parent better. Lmao. Again, i don't think that will go over well. He has goggles on whenever it comes to this kid and everybody can see it but him. Ughhhh.

ETA:

Well, I spoke to him. This entire conversation stemmed from a job schedule change where he would be potentially getting his son more often and asked how I felt about it. So I was honest. I simply said that I don't enjoy his company and listed all the reasons why (one which included when he hit my daughter with a stick hard af TWICE and his dad ignored it) every thought and feeling I had about any of it was completely dismissed and that it's none of my business how he parents (or doesn't parent) and that they're a package deal and he would cut off anybody who has an issue with his kid. "He acts completely different around me than everybody else" Well, perhaps that's why you're the only person who wants him around...? So, this is pretty gifty to me if I'm being honest. His dad has a long road ahead of him with this enabling, defensive mentality and I can't imagine a life like this with them. So, win win. Thanks for all the feedback though, it made me feel not so alone with it all.