Cause For Rash Only During The Morning Which Disappears In The Afternoon?

Hi my name is Stew and over the last three days my son has developed an itchy rash in the morning that disappears by the afternoon. It's school holidays at the moment so he has been sleeping in until about 10 am. When he gets up he has no rash, over the next few hours the rash appears but by afternoon its gone. The rash has round whitish raised inflamed lumps, some by themselves and others grouped together. Seems to start on his hands and spread over most of his body. I have taken some pictures if that would help. I can think of nothing that has changed to cause an allergic reaction, washing powder, food etc...
My son is 17 and we live in a rural part of Queensland, Australia.