I understand your worry.
And as per your description,possible differential diagnosis may include Supraclavicular swollen lymph node,even a type of torticollis (stiffening of the neck muscle).
As it's size is large and persisting more than 4 weeks so it's good to undergo further MRI,needle aspiration biopsy.
Even at the very least a chest X ray maybe done as well.
Also as he has eczema so any inflammatory condition can also result in swollen lymph node.
So further a clinical examination by a Dermatologist should be considered for proper treatment of eczema.
Hope this helps.
Positive feedback and bonus are highly appreciated.
Feel free for further query and follow-up.