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How old is he?
Did he use any new soap, lotion?
How large are the bumps?
His symptoms seem to be due to an allergy.
The cause is often not apparent.
How is he feeling now?
The distribution of itching can vary. Did he take any new food?
When did you give benadryl?
Shrimp is often a cause of allergy. Symptoms can appear after a few hours.
I hope benadryl & cortisone would help to reduce the symptoms.
You can repeat benadryl after 6 hours.
As the itching is decreasing, you can wait. If those do not help, please get him checked by doctor. He may need prescription grade antihistamine, steroid then.
As he is not having fever, it is unlikely to be due to other causes like viral rash, Henoch Schonlein etc.
How large is that blotchy rash?
Those can also be from allergy.
You have already used cortisone 10.
Triamcinolone Is more potent. If the bumps persist you can use it.
Let him go to sleep.