Ok well even if he does get enough income that he could live away from parents that doesn't necessarily mean he wants to or they want him to, it could be that all parties hope the dc staying at home means he can save up for a deposit on buying somewhere, and there's other reasons to stay at "home" too (and both parents houses ARE his home). Emotional support etc.
"but I had been spending days, nights and weekends there for at least 1 year." As is obvious from this discussion, that's not the same as living together properly combining finances, chores etc. It's still VERY fast for deciding to buy with someone! Even if there weren't children involved I'd advise against such haste.
"And what you are seriously lacking to get.. is that if he was my child moving in, it would be no problem" no! What YOU are failing to get is that the house you live in IS half HIS parents - exactly the same as if it were your child! The fact you say if it were your child there'd be no problem is precisely why you're clearly unreasonable! He has as much right to live in HIS FATHERS house as your child would to live in its mothers!
"Because it has been a stressful time for me. And I didn’t feel like sharing my life and struggles with anyone that I didn’t chose to!" But when you CHOOSE to be with someone who is a parent that's the breaks! If his child was also going through a stressful time when you were he would (rightly) prioritise the child's needs. This IS what you signed up for when you decided to be with a PARENT. Your needs will rarely come first for them. If you can't handle that then you're better off not being in a relationship with someone with kids.
Honestly the more you write the less I think you're mature enough to cope with this relationship, you'd be better off with someone closer to your own age who doesn't yet have kids...
...and that's without you facing up to the FACT that someone in their 50's who hasn't clearly said they want to is very unlikely to want more kids - another elephant in the room you're not addressing.
Re your post at 2242 "of course" it's a problem because it's not your child - you definitely need to end this relationship. You're far too resentful of his kids and should never have got together with someone with kids.
"He said that his children will always come first and that there should be no discussions to this.. which of course i understand" you very clearly DON'T understand that at all!
"If you actually read mu threads, I said I would never want him to kick out his son." Actually the reality is you DO want him to kick his son out, you just don't want to have to SAY it and look the bad guy.
"It is people like you who simply can’t deal with other people’s feelings that is what is wrong with the world!" Wow! No need for that rudeness!! Especially considering you couldn't give 2 hoots for your partners or his sons feelings!
Honestly if you really loved your partner you would WANT to get to know his kids! Because they are PART of him! They will ALWAYS be part of his life and always his priority.
You have a point re the chores/lack of financial contribution but honestly I don't think that's your real problem with him being there. You want your partner to yourself for the majority of the time and you resent that you can't because he is a father and that IS unreasonable and unrealistic - and was obvious to anyone who truly understood what it is to be with someone who is already a parent.