EDIT: Make sure your other financial business is unencumbered. Hopefully, you don't have any other joint debt with the boyfriend. If you do, work on separating these things, so you don't find yourself on the hook when your boyfriend decides he wants to buy things on credit for his lazy adult child.
For the time you are in the house, do not financially support these other people in any way. Pay only your portion of the rent and utilities. I'd think about cutting things like cable and internet now, and if it's so important to your boyfriend, let him have these things put in his name only.
Do not buy groceries or cook for these people. They should be doing that for themselves. Do not buy household supplies for them. Clean common areas only as much as necessary for your own sanity. Do not enable them in any way. If you do, you'll be supporting them forever, and probably in a very short time, you'll find yourself supporting a baby, too.
Take your valuables (cash, jewelry, important financial and legal papers) with you when you leave or get a safe deposit box. I would not trust this adult child or his new girlfriend having access to such things when you're not around.
I know this isn't what you want to hear, but you have to accept that this situation is NOT going to change. Save yourself now before it gets any worse.
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Is THIS how you really want to live? If the answer is "No," then you have to find out how to get out of this lease. It may even take some legal help to do it, but it would be well worth your peace of mind.
They---your boyfriend, his son, and the girlfriend---are comfortable living like this. You are not. They are NOT going to change, so you have to change the situation to save yourself and your peace of mind.
Start searching for affordable places to live. I don't know how much longer you're on the hook for the lease if you can't legally get out of it anytime soon, but get a plan together so you can get out as soon as possible.
If you still want to date your boyfriend from the comfort of your own place, that's your decision, but you would do well to ask yourself why you'd put up with this kind of treatment.