yes he should have spoken with you - but he has every bit as much right to have his son live with him as you do to have your daughters (that are not his).
Sorry - but if you live with a partner with children you cannot expect them to not parent because it is not for you. You take on their kids as he’s taken on yours. This should have been discussed before living together - but even then situations change.
When you move your partner into your house it is no longer yours singular. He isn’t a guest - this is his home equally with you.
And while I certainly understand it is not ideal having so many kids in one household - you together have 5 kids with another on the way. Having 4+ of them (all children) living together is not unexpected.
You aren’t unreasonable to expect a conversation, YABU to expect that this means you can exclude his son (while your daughters live there!) because it’s too much for you.
If you had moved into his home - would you accept him telling you your daughters can’t be there?
You can absolutely end the relationship. I would advise your partner to seek legal advice about his rights in a separation. You should likewise seek legal advice.
If you are considering termination, you would be best served by seeking competent counselling from a service that doesn’t push towards or away. And if you make a decision to terminate - you owe your partner a conversation (even if the decision is yours).