In this specific instance, I disagree with those who are pushing for something else "small." A fellow with a good shooting .257 Roberts, who is going to get one more rifle, doesn't need to step up to a 6.5, 7-08, or 7x57 (I love them all! There's not a thing wrong with them, but they are just not that big of a step up).
If I was buying for my son (and kind of wanted it to be a surprise, as the suggestions to just ask him are also valid), I think I would look long and hard for a FWT Stainless 30-06 with the composite stock. So he has the identical form-factor and functioning as his current Roberts, and a caliber that will not only fill in for the Roberts if need be, but also cover anything else he is likely to hunt. And, if the deer or pronghorn hunting weather is really nasty, he can leave that really nice Roberts at home and take the stainless/composite '06 that day.
Really no wrong answers, I'm just enjoying thinking it over,
Rex
