Not Tobias, just Toby.
DS has a name like Alexander or Nicholas. He's always Alexander, never Alex; always Nicholas, never Nick. We expect his school friends will call him the shortened version in the future but for now he goes by his full name.
Now we are expecting DC#2 and we really like Toby. Tobias is nice too but I can't see we would ever call him it. But would it be odd to have Alexander and Toby rather than Alexander and Tobias?
What do you think of just Toby?