Urinary Tract Infection (UTI): Although the patient does not report burning micturition, which is a common symptom of UTIs, the increased urinary frequency along with fever could still suggest an infection. The absence of dysuria (painful urination) does not rule out a UTI, especially in men, where the symptoms can sometimes be less typical.

Prostatitis: This condition, an inflammation of the prostate gland, can cause urinary frequency, fever, and sometimes systemic symptoms like itching due to an allergic reaction or a systemic infection. It's a common condition in young men.
Overactive Bladder: Although less likely given the acute onset and presence of fever, overactive bladder could cause urinary frequency. However, the itching and fever would not be typical symptoms of this condition.
Diabetes Mellitus: The patient does not report increased thirst, which is a key symptom of diabetes. However, some cases of diabetes can present with urinary frequency without significant polydipsia (increased thirst), especially if the blood glucose levels are not extremely high.

Sepsis: Although the patient's symptoms do not strongly suggest sepsis, any infection (like a UTI or prostatitis) can potentially lead to sepsis, a life-threatening condition. The presence of fever and systemic symptoms like itching should prompt consideration of sepsis, especially if the patient's condition worsens.
Kidney Stones: While kidney stones typically cause severe pain (renal colic), they can also lead to urinary frequency and fever if there is an associated infection. The absence of pain does not rule out kidney stones entirely.

Tuberculosis of the Urinary Tract: This condition can cause chronic symptoms including urinary frequency, but it's less common and usually presents with other symptoms like hematuria (blood in urine) and weight loss.
Interstitial Cystitis: A condition causing bladder pressure, bladder pain, and sometimes urinary frequency, but it's less likely given the acute onset and presence of fever and itching.
Systemic Diseases with Renal Involvement: Certain systemic diseases (e.g., lupus, sarcoidosis) can affect the kidneys and cause urinary symptoms, but these would be rare in a previously healthy 20-year-old male without other systemic symptoms.

Given the differential diagnoses, the initial approach should include:

Urinalysis to check for infection, blood, or other abnormalities.
Blood tests to evaluate for signs of infection, diabetes, or other systemic conditions.
Physical examination, including a genital and abdominal exam, to assess for signs of infection or other abnormalities.
Imaging studies (e.g., ultrasound) if kidney stones or other structural abnormalities are suspected.
Empirical antibiotic treatment may be considered if a UTI or prostatitis is suspected, pending culture results.

It's crucial to monitor the patient's condition and adjust the treatment plan based on diagnostic findings and clinical response.