Separation anxiety on another level

My son was born at the end of January so I’m hoping to get advice if anyone has been through it? My son who has been through the normal stages of separation anxiety has been staying with my mum (his grandma) one night a week for almost a year. It’s never been an issue. My dad (his grandad) has travelled, I’m not sure if that’s a reason but he’s literally not wanting my mum anywhere near him. She usually does bath and bedtime but for the last week, I’ve had to do the bedtime and wait for him to sleep before leaving. If she does it, he’ll scream the whole time. When he wakes up and realises he’s with her, the screaming starts again. She drops him to nursery and he literally wouldn’t get in the car, put himself to the ground screaming and crying the whole time. It was a worker at his nursery that had to come and calm him down. He doesn’t act this way with me and I’m not sure what to do about it. In his screaming he’s just shouting ‘mummy, mummy, mummy’. He’s recently learnt that he can call me and I’ll come, so if we’re playing he’ll say ‘mummy come’ and beckon me. I don’t know if that’s something to do with it.