Of course you have to welcome your SS into the home you share with his father. It doesn't matter that it was your childhood home - if your husband lives there with you it's his home too, and by extension your SS's as well. Please do not make either of them feel otherwise.
At one point I had three stepchildren, the partner of one of them, plus DH and our three DC in the house. I know how tricky it can be to find the right balance for everyone's sanity, but if you put the effort in it can be done. All my stepchildren stayed until age 26-27, because it's impossible to afford London rents unless you are half of a couple. So brace yourself for that and set some procedures in place for it.
I would start by talking to your husband about feeling left out of family relaxation time after dinner because he and his son watch things you don't like. Some sort of rota is needed here, where you take turns choosing what to watch, or what game to play, or in some other way find things the three of you can do. Something like:
Monday: you choose video for all
Tuesday: family games night (video games ok as long as they are suitable for you)
Wednesday: husband chooses video for all
Thursday: online pub quiz for all
Friday: You and DH watch something together while SS "babysits" your younger one, watching something with them upstairs, reading them stories and putting them to bed.
Saturday: boys night in the sitting room, you watch Disney movie with your little one on another screen
Sunday: SS chooses video for all
If I can give you some advice from long experience, it would be to focus on the enormous benefits of having your SS living with you. 1) Your DH no longer has to choose where to spend time and gets to have a normal family life with his son, which I'm sure is something he really treasures. 18 is a wonderful age to have a father on hand every day. 2) You get to develop your own relationship with your SS, not as an auxiliary to his father. If he is a good boy, as you say, and you get it working well, this will be a rich new relationship for you. 3) Your 4YO will get to have a real sibling relationship with their older brother, who probably didn't know them that well until he moved in.
My younger kids bonded much more strongly with the older ones when the older ones lived with us. Babysitting them, playing video games with them, helping them with piano practice, homework or computer problems, hearing the dinner table talk, etc. brought them close in a way they never were before and that was the biggest gift of all. The best way to foster that bond is to get the SS to help with the little one - to do some homeschooling with them, take them to the park to kick a ball, or babysit while you and DH go out for a long walk, so they have that sense of nurturing and responsibility. And that's really only going to happen if he lives with you.
So I say embrace it and make the most of it. And don't let them exclude you - speak directly (but not accusingly) to your husband about feeling left out.