TL;DR: This is Ewing sarcoma because it is a young boy with small blue cell bone tumor in the diaphysis of the femur

Ewing sarcoma is a malignant bone tumor common in young boys <15y.o. typically found in the diaphesis (middle) of long bones (particularly femur) or pelvic flat bones. It is a small blue cell tumor, meaning the descriptor of small, uniform, hyperchromatic cells with high nucleus:cytoplasm ratio. It also has an "onion skin" periosteal reaction in the bone (described as tissue surrounded by concentric layers of reactive bone in the question). Not mentioned is the association with t(11;22). [FA 2020 p 464]

Why the others are wrong:

• Chondrosarcoma is a tumor of malignant chondrocytes (catilage) and found mainly in the pelvis, proximal fever, and humerus
• Eosinophilic granuloma is is a rare, benign tumor-like disorder characterized by clonal proliferation of Langerhans cells and the most common varient of Langerhans cell histiocytosis.Would likely say something about Birbeck granules in the stem and would have associated symptoms such as rash [FA 2020 p434]
• Multiple myeloma is a plasma cell dyscrasia [FA 2020 p431] with overproduction of (typically) IgG. Characteristics include CRAB - hyperCalcemia, Renal involvement, Anemia, Back/Bone pain due to punched out lytic bone lesions (typically of the spine)
• Nephroblastoma/Wilm's Tumor is the most common renal malignancy of early childhood and presents as a flank mass [FA 2020 p 606]
• Osteosarcoma (or osteogenic sarcoma) is a primary bone tumor common in men <20. It is typically found in the metaphysis (tips) of long bones. Histology shows pleiomorphic osteoid producing cells. On X-ray you would potentially see a Codman triangle (elevation of periosteum) or sunburst pattern
• Small lymphocytic lymphoma is essentially the same thing as chronic lymphocytic leukemia. It is considered a non-Hodgkin lymphoma of B-cells and the most common leukemia of adults (as CLL). Smudge cells would likely be seen on peripheral blood smear. It would not likely present as an osteolytic lesion and is unlikely in children. [FA2020 p432]