I don't raise Red Orpingtons (but aren't they gorgeous!). I do have several varieties in English Orpingtons though. The cockerels usually develop a bit slower than hatchery stock you typically see her in the US. I'd say at ten weeks you have a very handsome cockerel. He'll be stunning once he's all filled out. I just love mine.
I could be wrong and it might be a pullet, but I'm really thinking cockerel at this point. 