Hi R.,
Girl, I feel for you! I really don;t know what to say, you pretty much summed it all up in your request.
Talking to her probably won;t help much, because if she is going to be with you 3 more months, she isn't going to ignore your babies cry for her & she isn't going to stop being grandmaw & ditch all the hugs kisses & cuddles.
BUT you could tell her that when you are home, whether he cries for her or not, she needs to send him to mommie. Four hours a day isn't long for her to watch him, but with her living there she does have access all day long to her grandson whom she has attached herself too & him to her, because she is there around the clock. Im sure it was hard for them to give up their home and friends to come to you. He is probably what has kept her "sane" from it all.
But on the other hand, you are his mother & a line does need to be drawn. The longer they live there, the more attached they both will become. My suggestion is explain in detail your hurt & concerns but also let her know that you realize they have up their life for yours, because basically they did. I would tell you mom that the longer she stays the more hurt she will endure if she waits 3 months to move out, because of her attachment to the baby. Make it 2-sided rather than one, I think I would involve your dad in this conversation also. I'm sure he is ready to have his own place. When they move out, I would suggest you take a couple months leave from school & work & rebuild your relationship with your son. Because if you don't & you drop him off at her house to babysit, you're gonna have a fight getting him back home if he already cries for her even with you there.
THEN after you have severed their closeness a bit you can gradually take him to her to babysit, but don't leave him no matter how much he cries.
It isn't the four hours a day, 3 days a week & the Saturday class that's brought this on, it's the 24 hr access to each other.
If they agree to go ahead & move & you cannot take off work now nor miss those Saturday classes, let her keep him at her house & you pick him up asap. (the transition may be easier on your son if he has to go to a strange house to stay with your mom, rather than she come to your house to keep him)
Your parents surely know that as attached as your son is to her, it is going to be hard on him when they do move out & on your mom too, so with that being said, I would insist they go ahead & move out, to ease the pain of them both, but if she insist on staying, then I would insist for the best of your son, before he is attached any deeper, that they move. She should love him enough to realize it's the best thing for him, & for you & you and you son's relationship.
Just out of curiosity, what does your husband say about all this? Isn't he ready to have his home & family back?
If it doesn't work to where they move asap, then you might want to prepare your son now, by putting him in daycare 3 days a week. With the understanding to your mom that you are beginning the break away transition for your son so that it will be easier on him when they move. I am a mother & a grandmother, so I feel for you both, BUT YOU ARE THE MOTHER!! & You need to remain the mother, you are the one that has to step up to the batters plate & pursue whats best long term for your son. And YES it is going to hurt your mom, but its going to hurt your son worse if you don't get a handle on this. She needs to be in her own house so she can be grandma, not in your home being mommie & grandma, she might get her feelings hurt, but she wlil get over it. you explained it very well to us & I think you should explain it that way to her & your dad with alot more add-ons.Gentle but firm. Let her know how much you appreciate what they did for you, but that your son has to know your the mommie & she's grandma & he wont learn that as long as she is playing both roles.
Best Wishes to ALL of you & please update so we know the outcome :)