Your son did a lot of fun things with your mum, and they both said they had a good time, so maybe when he said "She doesn't want me" what he actually meant was "I wanted to stay longer, do more fun stuff, eat more ice cream, but she brought me home."
When your mum said "until Saturday evening" her thinking may have been "I said I'd cook Son and GF a meal Saturday evening so I can have dgs until then" without thinking about the time it takes to turn around between the childcare and the cooking. In her head evening; in practicality early evening start cooking, late afternoon return home, early afternoon take dgs home.
Have you spoken to your mum since? Properly spoken? Asked her how it went with your son, how she coped, what she found difficult, any questions she may have about his routine or the way you do things? Have you asked her how she would feel about having him for several days while you're in hospital, bearing in mind that there is a possibility you may need to stay in longer if baby has jaundice, or any other complication should arise? Might it be easier of she came and stayed at yours while you are in hospital so ds has his own bed, toys, books, entertainment etc? Or could you offer to send him with a collection of dvds, game console if he has one, tablet if he has one. Tell her it's ok for him to have a lot of screen time if she needs the break from constant physical or mental activity.
Good luck, OK. I hope you get something sorted.