In your case, I would probably keep the bird until it either crows, mates with females, or lays an egg. I think it looks more like a cockerel than a pullet to me, but it's easy enough to be wrong. Given that it's already 15 weeks old, I don't think you'll have to wait all that much longer to be...
If the father was Rhode Island Red and the mother was the Sussex, then you should only get red pullets and white cockerels. But you seem to have a red cockerel, so something isn't right.
If someone raised sexlink pullets (red) and sexlink cockerels (white) and crossed them to each other, you...