ggmarion, I was desperate to knit something for first gc three years ago. DiL is part of an extended family who pass around babyclothes as needed, & modern mums seem to be very wary of washing handknit items. We agreed I could knit one "old fashioned" matinee coat & baby would be photo'd in it for grannies boasting book! I also knitted a very plain wrap-round style cardigan which DiL liked. I have the photo, & saw him wear the cardi 2 or 3 times.
You could trawl through internet sites which have free downloadable patterns, borrow patterns from friends, or check craft books in local library. Discuss things with Dgd & if you get the impression she's not that fussed just knit something - balls for a hanging mobile perhaps (!?) - anything, just to express your love for the newcomer.
He's a summer baby, so I might be inclined to go down the route of knitting colourful Christmas hats/cardis/toys etc.,etc. Your choice, & great fun for Baby's First Christmas photos.