Viral Exanthem: This is the most likely diagnosis given the age of the child, the presence of a rash that comes and goes, and the intermittent fevers. Viral exanthems are common in this age group and can present with a variety of rashes and fever patterns. The use of ibuprofen and Benadryl by the mother suggests an attempt to manage symptoms, which is consistent with a viral etiology.

Roseola: Characterized by high fevers followed by a rash once the fever resolves, roseola is a common viral illness in young children. The rash typically appears on the trunk and can spread to other areas.
Urinary Tract Infection (UTI): Although less common in boys, UTIs can cause fever and nonspecific symptoms in young children. The rash could be unrelated or a sign of a more severe infection.
Pharyngitis: Caused by viruses or bacteria, pharyngitis can lead to fever and rash in some cases, especially if caused by Group A beta-hemolytic streptococcus.

Meningitis: Although the presentation does not strongly suggest meningitis, it is crucial to consider this diagnosis due to its high morbidity and mortality if missed. Symptoms can be nonspecific in young children, including fever, irritability, and rash.
Bacterial Sepsis: Sepsis can present with fever, rash, and nonspecific symptoms. Early recognition and treatment are critical to prevent severe outcomes.
Kawasaki Disease: An acute febrile illness of childhood characterized by fever, rash, conjunctivitis, and changes to the lips and oral cavity. It is a leading cause of acquired heart disease in children and requires prompt diagnosis and treatment.

Juvenile Idiopathic Arthritis (JIA): Can present with fever and rash, although it is less common in this age group. Systemic JIA is a subtype that includes fever, rash, and other systemic symptoms.
Autoimmune Hemolytic Anemia: Although rare, this condition can cause fever, rash, and other systemic symptoms due to the autoimmune destruction of red blood cells.
Leukemia: Presentation can include fever, rash, and other nonspecific symptoms. Although rare, leukemia is a critical diagnosis to consider due to its significant implications for treatment and prognosis.