Girl or boy? Sue’s prediction:

cockerel (male, 14 weeks or so)

This young cockerel is just starting to get his adult feathers. It's important to carefully examine the feathers over the back to see their shape, especially if they're one colour – white pointed feathers can be especially difficult to see. Check to see if the newer feathers (which may still have a waxy sheath at the base) are longer, pointy, with shiny edges compared to the more rounded juvenile feathers underneath, which have yet to moult. If so, it's a male. If the new feathers are rounded and not shiny on the edges, it's a female (pullet).

I'd also expect by this age, a cockerel would be slightly taller and lankier than his sisters.