Matthew (Clear Vision), the law has gone daft if you can't take your nephew out for a few days work, aged 15, as long as he doesn't do anything dangerous; such as work at height.
It could also be classed as work experience, which many people of his age do already.
Just don't pay him anything; honest (
), since it is work experience. I think this would get you out of any tax/ni complications also.
I'd teach him how to clean windows for an hour or two on your own house, before loosing him onto your customers.
He'll no doubt feel a bit self-conscious, so a bit of practice in a 'safe environment' would be best to start.