Can I anyone give advice for me about a disrespectful 21 year old son?

Forgive me for this being lengthy

So I have searched Reddit about this and I guess it seems to be quite common. I feel like, there's something in the water. TikTok maybe? Its as if something is brainwashing these kids. I've been having issues with my son since high school, around maybe 17. But issues started possibly around 16. We've (my wife and I) have never really had discipline issues with him. I've never really harshly disciplined my son because according to IEP, he's on the spectrum, so we thought if we ever got around to spanking him that he wouldnt grasp the concept and respond in a way that it was violence. I had already had to smack him one day cause he took a swing at me because I had repeatedly gotten after him about washing dishes, which led to an argument again, he said something disrespectful and I had it, so I delivered a swift kick to the butt, he took a swing at me so I clobbered him. I think that was the most in long awated discipline. Probably the wrong way but.. His mother and I had a rocky first 10 to 15 years of occasional bickering and shouting matches, which I know probably did not help, but my oldest at 26 faired fine. But I feel, my 21 year old just does not get it.

Just a few months ago his girlfriend's mother kicked her out due to other circumstances. She has one of those mothers who can't seem to grow up and is constantly getting her I assume drug fixes so basically her daughter played the role of a mom to her younger siblings.

So I have finally gave him till Nov of this year to find a place. From disrespect talking to even almost ruining our vacation because all I asked him is not to watch horror movies around our youngest child. If better behavior, I'd give him till Dec. When I want to sit him down to have a civil discussion, he immediately puts up a defense with a chip on his shoulder.

I have another old adult child , almost 30, from another relationship and she is bitter about the split up between her mom and I. She talks to my son and I am starting to fear she is putting ideas into his head. Or it could social media.

My wife and I are at our witts end.

One of the other things I am concerned about is what leverage does he have? We really don't want to kick him out, but then that could be our mistake. At this point I don't want it to turn into trying to evict squatters, especially his girlfreind, which was probably another mistake taking her in, she has an avenue with her grandparents. But I feel she is not helping the situation despite the fact she's sweet and helps out around the house when she can. Basically, I think she's caught in the middle.

My son has a job but concocts reasons for not being able to pay rent. Getting him to be responsible in stuff like: Throwing his own trash away, doing his laundry but respecting ours by not just dumping it back into the laundry basket, cleaning up after himself after making food, eating food, etc., chores around the house and trying to get him to do anything is pulling teeth. He's been butthurt lately because I will not let him on my computer to play games he's bought, but I had told him some ground rules before he decided to buy a couple games and install them, but as always, he broke those rules so now I will not let him on unless he agrees to sit down with me to talk about the ground rules again, that is where he gets defensive, we end up in a shouting in a match because he insists on interrupting and being an asshole. I am trying not to get to the point of punching him and knocking his teeth out.

We have given him the ultimatum of joining the military or get out. I have even told him about evicting him since he is 21. He just has this full on entitlement syndrom and insists we owe his something, and I have made it clear that we don't. I have even bought a small car which I used for commute and had told him a couple years ago it would be his if he can be responsible.

sorry for the length but seems I am not the only one with this issue and I have read others situations