It's usually within the first few hours of sleep I think. He's always covered in a layer of sweat, such that if I run my hand across his back, my hand is wet. It doesn't wake him, nor does it need a change of sheets, although it makes the sheets feel damp (but dried by morning). Have googled and scared myself silly about causes of night sweats 
Has anyone else similar experience or can reassure?? If it's relevant, he has not yet hit puberty (very small for his age) .