I am sure you will find that almost everyone here says the same thing.
The baby will get used to the changed regime, even if it does take a few sessions of crying and sobbing.
Keep your cool, just comfort him (or if he refuses with the old "arched back" trick, just leave him to cry in a safe place if he won't be distracted.
Of course he wants Mummy there all the time, she's his safe haven and some of his distress is fear.
He needs to learn now, with a loving, caring grandma, that he can be safe and happy away from her. It won't kill him to cry for an hour; though it will break your heart to see it.
If you both give in now, he is learning that all he has to do is scream and Mama comes running. Stick to your guns and he will learn a valuable social lesson which will benefit you all.
This weekend, I "babysat" for a 10 year old whose mother's obsessive need to give her son everything he wants, including all her attention, is turning him into a selfish, arrogant and very unhappy boy.
Had she put her foot down when he was younger, he might not be the disrespectful, demanding brat he is now.