What is the most likely diagnosis for a 10-year-old boy with a non-healing rash and fluid-filled blisters after being treated for contact dermatitis?

The diagnosis of impetigo is consistent with the presentation of a non-healing rash and fluid-filled blisters, especially following an initial treatment for contact dermatitis. Impetigo is a bacterial skin infection that is commonly seen in children, often manifesting as crusty lesions and blisters. The fluid-filled blisters can result from the infection that develops in areas of broken skin or existing lesions, which in this case may have been initiated by contact dermatitis.

In contrast, conditions like eczema typically involve chronic skin changes and may not specifically present with fluid-filled blisters in isolation; rather, they are characterized by dry, itchy patches. Psoriasis usually presents as well-defined red plaques covered with silvery scales and doesn't typically follow contact dermatitis treatment. An allergic reaction could be a possibility but would usually include a more generalized response rather than localized blisters and would generally resolve with the cessation of the allergen exposure.

Therefore, given the boy's symptoms and the context of prior treatment for contact dermatitis, impetigo stands out as the most likely diagnosis.