It looks like barring to me.
A chicken can have both barring and mottling, but I have never heard of just mottling making a bird look barred like this. When he grows up, breeding him to some hens and looking at the offspring might settle the question of whether he has the actual barring gene or just something that looks like it.
You say she does not have any barred birds. The barring gene is also found in Cuckoo and Crele colors, and in all variations of them (like Lemon Cuckoo and Lavender Cuckoo), and in any autosexing breed (like Bielefelder and Rhodebar and Cream Legbar).
Some white chickens also have the barring gene, but of course white bars don't really show on a chicken that is white all over.
If you have a complete list of the available breeds, it might be possible to pick out what one gave barring. Otherwise, we're just guessing, and may never know for sure.