Hello. Welcome to Ask The Doctor.
I have read your query.
I have seen the attached photograph also.
As per details the most likely possibility for your son's symptoms is that he has got some insect bite.
It is also known as Pedrous dermatitis characterised by sudden onset of rash usually localized to an area. The central black crusts represents the site of bite which later gets crusted.
Pain and swelling in the the area is the result of inflammation incited by the antigen injected into the skin by insect.
In such cases, I prefer to advise topical antibiotic cream like mupirocin 2 percent over the affected area followed by a combination of antibiotic and steroid like hydrocortisone cream.
For pain and swelling , syrup containing ibuprofen helps.
When severe, oral antibiotics are also added.
You should keep the area clean and hygiene should be maintained.
Visit your doctor so that he can clinically examine the child and give you prescription.
You can share my opinion with him.
I hope it helps.
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