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Blisters on the glans penis of a young boy could be due to injury or due to fungal infection. Usually fungal infections are more associated with itching.
Rarely, scabies can present as bumps under the skin in the pubic region, and small blisters could also be caused by allergies or yeast.
It is very unlikely for sexually transmitted infections, including herpes to present in such age group and with a single blister.
You may consult with your pediatrician for a proper examination and treatment for the same.