A 3-year-old boy from North India presents with progressive muscle weakness, difficulty climbing stairs, and frequent falls. On examination, he has pseudohypertrophy of the calf muscles, a waddling gait, and positive Gowers' sign. Serum creatine kinase (CK) is markedly elevated at 8,500 IU/L. Genetic testing reveals a deletion in the dystrophin gene on the X chromosome. What is the most likely diagnosis?