Tinea Corporis (Ringworm): Given the description of the rash as itchy, not resolving with antibiotics, and having a herpetiform appearance with lesions slowly moving centrally, tinea corporis is a strong consideration. The fact that it was initially diagnosed as folliculitis but did not respond to antibiotics further supports this, as fungal infections do not respond to antibacterial treatments.

Contact Dermatitis: This could be a possibility if the patient has been exposed to a new substance that his skin reacts to, causing an itchy rash. The herpetiform appearance could be seen in certain types of contact dermatitis, especially allergic contact dermatitis.
Nummular Dermatitis: Characterized by coin-shaped patches of dermatitis, this condition can be itchy and might resemble the described rash, especially if the patient has dry skin or has recently experienced skin trauma.
Psoriasis: Although less common in the description provided, psoriasis can present with various morphologies, including plaques that can be itchy and might resemble the herpetiform lesions described.

Lyme Disease: Caused by Borrelia burgdorferi, transmitted by tick bites, Lyme disease can present with a characteristic "bull's-eye" rash (erythema migrans) that expands centrifugally. Although the rash described doesn't perfectly match, Lyme disease is a critical diagnosis not to miss due to its potential for serious complications if left untreated.
Cutaneous T-Cell Lymphoma (Mycosis Fungoides): A rare form of skin cancer, mycosis fungoides can present with a variety of skin lesions, including patches, plaques, and tumors. It's essential to consider this diagnosis, especially if the rash is persistent and not responding to typical treatments.

Erythema Migrans (without the typical bull's-eye appearance): While mentioned under "Do Not Miss," it's rare for erythema migrans to present without its characteristic appearance. However, it's crucial to keep in mind, especially in endemic areas.
Granuloma Annulare: A benign skin condition characterized by bumps or lesions that form a ring pattern. It's less common and might not perfectly fit the description provided but could be considered in the differential diagnosis due to its ring-like appearance.
Pityriasis Rosea: Characterized by a herald patch followed by a secondary eruption of oval-shaped lesions, pityriasis rosea is another rare consideration. The description doesn't perfectly align, but it's a condition that can cause itchy skin lesions.