Yolk Sac Tumor: This is the most common testicular tumor in children, especially in those under the age of 2. Although alpha-fetoprotein (AFP) is typically elevated in yolk sac tumors, it can be normal in early stages or in some cases. The tumor's heterogenous echogenicity on ultrasound and the patient's age support this diagnosis.

Teratoma: While less common than yolk sac tumors in this age group, teratomas can present with variable echogenicity due to their mixed tissue composition. Normal tumor markers do not rule out teratoma, as they can be non-secreting.
Epidermoid Cyst: These benign tumors can display heterogenous echogenicity and typically have normal tumor markers. They are less common but should be considered in the differential diagnosis.

Leydig Cell Tumor: Although rare in children, Leydig cell tumors can produce estrogen, leading to hormonal imbalances. However, in this case, estradiol levels are normal, making this less likely. It's crucial not to miss this diagnosis due to its potential for hormonal effects and malignancy.
Gonadoblastoma: This rare tumor is often associated with gonadal dysgenesis and can be a concern due to its potential for malignancy. Normal tumor markers and the absence of mentioned genetic syndromes make this less likely, but it's a critical diagnosis not to overlook.

Sertoli Cell Tumor: These tumors are rare and can present with variable ultrasound findings. They are typically benign but can be malignant in rare cases.
Rhabdomyosarcoma: Although more commonly associated with the paratesticular region, rhabdomyosarcoma is a rare but aggressive tumor that could potentially present as a testicular mass. The clinical context and imaging findings would be crucial in suspecting this diagnosis.
Other Germ Cell Tumors (e.g., Embryonal Carcinoma, Choriocarcinoma): These are less common in children and often have elevated beta-hCG or AFP, which are not present in this case, making them less likely.