@bwise1966,
bwise1966 wrote: i'm just his Mom's boyfriend.
is that how he and his mother see it?
In any case, get yourself to counselling - along with your partner.
If you are co-owner of the home you will be living in, you have equal rights to decide who lives in your home.
I would say that unless he's contributing to the household in some way, it is time for him to move out.
___
What will you do if your girlfriend again chooses her son over you? is it a deal-breaker?
I suspect your girlfriend and son know that it isn't a deal-breaker for you.
You were already upset about this a year ago - and it seems that the only thing that happened was that you got more involved in the bad family dynamics - without gaining any rights for yourself.