I wish you the best! I don't have much to suggest, my son is twelve and until this summer I had the same problem/s with my mom. Except my son was at her house everyday during the week, because he went to school in her town, so she would pick him up from school. My mom had "withdrawls" when my son started staying home this summer and not going to her house, she even called him everyday. And he's not going to school in her town this year, he's going in the town we live in (long story as to the school thing) and I'm pretty sure my mom will survive. LOL!
I don't know how many daycares are in your area, but it is an option to take him to a daycare farther away from her house?? Maybe if it wasn't so convienent for her she wouldn't drop by everday. IF you do decide to do something like that, just make sure you explain to the daycare provider WHY you are taking your son somewhere else. Being that she isn't exactly kosher with your mom being there everyday, I'm sure she will understand.
My only advice is confront her now and try to get it resolved now or else you'll be like me and deal with it for 12 years!! And that is not fun. I battle my mom all the time, because from the day he was born, she thinks SHE is his mom and after 12 years, sometimes I think she still thinks that.
Again, you have my prayers and I wish you the best. Its a tough situation, espically when you are living in their home (we did that too for about a little over 3 yrs total) so I really feel for you.
If you ever need to vent, talk or just need someone who knows how you feel just to listen, feel free to mommymail me anytime.
I may not have any suggestions, but I will be a sympothetic ear.
LOTS of HUGS!! K.
Oh, one more bit of advice - if you don't want her dropping by YOUR house everyday, don't live too close! :O)