Irrespective of how old the OP's son is... he's still her child. She's still going to worry about him and feel that instinct to protect him, or provide for him if she thinks it'll help him, because she's his parent.
Having said that, however...
OP, your son has been an adult for the last 6 years now. More to the point, he has a child of his own now. A child whom he needs to be not only providing for, but also setting a good example for them to model themselves upon when they grow up. If you allow him to "come home" (...really?! His home, surely, should be with his girlfriend and their child, not bunking down in separate homes to one another if they're still together, that is) then you aren't helping him. You're allowing him to continue to behave like a child - and you're showing your three younger sons that they can behave just like their big brother, because you'll always fund such behaviour. Is that what you want for your younger sons? A lifestyle like your older son's is?
Presumably, because you mention that he pays rent to you and his father, your oldest son is working. He's setting a good example to your younger children by showing them that he has a job, that irrespective of the fact he still lives at home (as another poster said, a lot of 20-somethings without the responsibilities of being parents themselves still live with their parents... and a lot don't), he accepts that he's an adult, and behaves accordingly.
If I were you, OP, I'd not allow your second-oldest to return "home", as you put it. He needs to stand on his own two feet and figure out how he's going to provide a roof over his head... and for his own child at the same time. As soon as that baby was born? Your son became a father. And with that... as I know you're aware... comes certain expectations of behaviour. He's reverting to wanting to be a child again, I'm afraid - and that's not an option for him now, is it?
I'd also want to know if he and the girlfriend are still together - and why he wasn't able to stay with her and their child, if they are still together. Something doesn't sit too well about this separation. Especially as you say that he has "an overbearing personality" (and yes; as others have pointed out, that smacks of being allowed to get away with being rude whilst he was growing up, so that now it's second nature to him to be "overbearing"). Your oldest doesn't want to share a room which he pays rent for... and he doesn't have to. He shouldn't have to, or be expected to, just to suit a man-child who refuses to grow up and accept responsibilities which he helped to create.
Also, 5 children's nothing out of the norm, I'm afraid. My father is the oldest of 8, and one of my friends has 10 children. My grandparents provided for their children themselves (my grandmother was a single mother for a while back in the days when it was socially stigmatised, too, whilst my grandfather went off and had more children with other women...), and my friend and her husband have never received any benefit other than Child Benefit for their brood. My grandmother always said that the only thing which made my grandfather stop behaving like "a permanent schoolboy" was joining the Merchant Navy. So your son's attitude to life isn't anything new, I'm afraid. But he has to grow up and provide for a family which he, not you, created.
Good luck.