Vitamin B12 Deficiency: This condition can cause numbness in the hands and feet due to its role in the production of myelin, the fatty substance that surrounds and protects nerve fibers. The symptoms, combined with the boy's age and the provided lab values (which do not immediately suggest another acute condition), make this a plausible diagnosis. Vitamin B12 deficiency can also lead to megaloblastic anemia, which might explain the slightly low hemoglobin and hematocrit levels.

Hypocalcemia: Although the phosphorus level is elevated, which could suggest secondary hyperparathyroidism as a response to low calcium levels, the direct measurement of calcium is not provided. However, numbness or tingling in the hands can be a symptom of hypocalcemia. The elevated phosphorus could also be related to other conditions, such as kidney disease, but the creatinine level is within a relatively normal range.
Diabetes Mellitus: Early stages of diabetes can cause neuropathy, leading to numbness in the hands and feet. However, typical symptoms of diabetes, such as polyuria, polydipsia, and weight loss, are not mentioned. The provided lab values do not directly support this diagnosis without additional context, such as glucose levels.
Thyroid Disorders: Both hypothyroidism and hyperthyroidism can cause neurological symptoms, including numbness. However, specific thyroid function tests (TSH, free T4, free T3) are not provided to support or refute this diagnosis.

Multiple Sclerosis (MS): Although less common in children, MS can present with a wide range of neurological symptoms, including numbness. It's crucial to consider MS to avoid missing this potentially debilitating condition.
Guillain-Barré Syndrome: An autoimmune disorder that can cause rapid-onset muscle weakness and numbness, often following an infectious illness. It's a medical emergency that requires prompt recognition and treatment.
Kidney Disease: While the creatinine level is 0.61, which might not immediately suggest severe kidney disease, the elevated phosphorus level could indicate some level of renal dysfunction. Kidney disease can lead to a variety of electrolyte imbalances and secondary conditions that might cause numbness.

Wilson's Disease: A genetic disorder that leads to copper accumulation in the body, potentially causing neurological symptoms, including numbness or tremors. It's rare and typically presents in younger individuals, but the diagnosis requires specific tests (e.g., serum ceruloplasmin, 24-hour urine copper).
Refsum Disease: A rare genetic disorder affecting the metabolism of a specific fatty acid, leading to the accumulation of toxic substances that can cause neuropathy among other symptoms. It's extremely rare and would require specific diagnostic tests.
Fabry Disease: Another genetic disorder that results from the buildup of a particular type of fat in the body's cells, affecting many parts of the body and potentially causing numbness or pain in the hands and feet. It's rare and would need specific diagnostic tests for confirmation.