Yeah i suggest the same as emma maybe if she was cremated put some of her ashes in a gift for him to take with him. Can get them in all sorts these days x
Sometimes respect his desire to not do soemthing but still commerate with a gift, you can pay to get those images painted where it’s like a portrait of grandma and the twins together, I’ve seen online and it’s real cute!
I’m sorry for your loss ❤️ Did she have a favourite place to go/thing to do? Could maybe go there, eat some birthday cake and chat about fond memories? x