I would be looking after the needs of my OWN children first, in my OWN home.
It is his child, but as it is YOUR house, this really should have been discussed beforehand.
What was his plan by the way? Where was he going to sleep? How would this possibly work in a 3 bed house with 3 DC there already, yourselves and a baby on the way?
You are not responsible for him and the other parent being shitty parents and not controlling their children to the point they become menaces to society. That is on him and his DS's mother to navigate.
You are right unfortunately. Children ultimately come first and if his DS needs somewhere else to stay then your housemate needs to move out and find somewhere for him and his DS to live.
It doesn't need to necessarily be the end of a relationship, as many couples live separately successfully but I definitely would be assessing if it is worthwhile continuing a relationship with a man with whom you have major clashes with in regards to parenting. You do after all have 4 children of your own to consider!
In terms of the pregnancy, it depends how early on it is and your own personal views. I know that me personally, I couldn't go through with an abortion, but if it seems like the right option for you, then this is something to consider (but not lightly either!). At the end of the day, from what you have said, you will basically be a single parent to 4 children, even if he is "involved". Thats a lot of stress and work. I wouldnt blame anyone for wanting to terminate.