Give him some anti histamines and see if it goes down
Thani for the comments of help it seems to have gone down very well today just has an outline on his arm
Our little ones went like this too, she also came out in a rash on her belly 7 days after but other than it being itchy she has been fine in herself, dr said all normal c
You could always pop to the pharmacist today and ask? I think ones in big supermarkets would still be open on a Sunday.