I'm a bit late for this post and early in the parenting world, but here is out experience until now.
We are a mixed EU family with a 2 year old kid, we each speak our native language with the kid and English with each other. It's a interesting setup, but it works for now.
The child has dual citizenship and we try to visit the other half of the family as much as possible, as well as speak to them via WhatsApp.
He's to small to understand things like culture and customs, but my wife and I are pretty passionate about these things so I think in time he will catch on to it. We also have tons of books in both languages, so we're reading him the stories that both parents grew up with.
We can't control to what nation he will feel closest, but we can show him his roots, family and national history.