Yup. I can sympathise here. I went through pre-natal depression with my two, and my mum never offered any practical help, she just told me how bad SHE was feeling. When my sister's husband got taken into hospital with a serious heart problem, I had to travel with my dd on the train to help out with her 4 children, whereas my mum, who only lives around the corner from her, never offered to help. She just said "Well, she doesn't have to visit him every day does she?" My other sister also has 4 kids, one with Downs Syndrome, and my mum has never babysat for her and she won't even look after one of the other children overnight whilst her child is in hospital.
Now I am planning a move to France and she hasn't asked how it's going, why I'm moving or anything about it. She simply doesn't care. Yet to everyone else she's a saint who fosters kids and brings up my brother who has learning difficulties. They don't realise that the only reason she has foster kids is so that she doesn't have to look after her own grandchildren (she gets more money and more praise for having foster children), and yet whenever she goes on holiday my poor brother is palmed off either on myself or my sister with the 4 kids (one with DS).
Sorry to rant a bit, but I totally understand why you are so peeved off and I'm afraid I have no advice for you. My mum hasn't changed, she's just got worse. All I can say is, don't expect much and don't rely on her emotionally, or you will be continually let down. Explain to your ds too that sometimes people say they will do things, but then they will forget. He will have to get used to this anyway in life, just make sure that he doesn't rely on her every word either. Keep her at arms length if you can, but don't stop her from coming. That way you are keeping the door open, but preventing yourself and your family from being hurt.