Is it small ish spots, like a heat rash type thing or larger 'blotches' like hives/nettle rash? If the latter then it's urticaria, common in children with viral infection and it keeps disappearing and re-appearing in various places.
If the former as a PP said, does it go white when pressed with a glass? If not then call a doctor/NHS direct etc. is he unwell, complains of headache, aversion to light etc?
It's all over his back and chest, blotches on his cheek. It doesn't feel very rough, doesn't dissapear under a glass. It's only a slight rash, not instantly noticeable. He's poorly from whatever virus he has, tired, hot and has a cough. He's not moaned about the light though.