Your son may be having impetigo on neck.
It is bacterial infection caused by staphylococci.
There may be scab on the lesion. He may be having pain.
As yuo said, he is having atopic diathesis .
So, he is having tendency to have bacterial infection like impetigo.
Give him oral antibiotics
in paediatric dose.
Give him anti inflammatory drug.
Give him dettol liquid bath.
Improve his hygiene and immunity.Consult dermatologist
I hope you got my answer.
Dr. Ilyas Patel MD