In a 9-year-old boy with progressive weakness and a family history of similar conditions, what is the most likely diagnosis?

In this scenario, the details provided can be aligned with a diagnosis of Duchenne muscular dystrophy (DMD). This condition typically presents in boys around the age of 3 to 5 and is characterized by progressive muscle weakness, which aligns with the child's symptoms. The family history of similar conditions is significant, as DMD is an X-linked recessive disorder and often affects multiple males within a family.

Becker's muscular dystrophy, while also a type of muscular dystrophy, typically has a later onset and milder progression than DMD. The child being only 9 years old with progressive weakness strongly suggests DMD due to its earlier onset and more rapid progression.

Cerebral palsy primarily affects motor function and muscle tone due to brain injury and does not have a hereditary pattern like muscular dystrophies. Spinal muscular atrophy, while also a genetic condition causing muscle weakness, typically presents with different characteristics and often leads to significant respiratory issues, and could be more common in younger children.

Thus, given the age, symptoms, and hereditary background of the condition, Duchenne muscular dystrophy is the most likely diagnosis for the 9-year-old boy.