I think I'd leave it as a surprise. I also wouldn't want a surprise house guest, in general (really don't want ANY house guests surprise or no lol), but I can think of some people where I would be totally ok with that happening because I love them so much and I know they aren't judging my cleanliness or readiness and I have to imagine a grandchild falls in that category.
You could even volunteer to make a grocery run after you show up if you think she'd be worried about that.
But two Christmases ago I surprised my mom by showing up at her house and it was SO SO great.
I’m very slightly annoyed when a package I ordered myself arrives unexpectedly early so I definitely wouldn’t be excited for unexpected guests lol but my family’s love language is planning and boundaries so this is never an issue.
I would be open to him staying with you and stop by with him or surprise them at your house and say he’s in town for the week, he’s welcome to stay with you unless they’d want him with them.