Yes, I DO agree the diagnosis done by specialist not by physiotherapist as each one would help and aid in proper diagnosis and finally clear prognosis!
however may here I can say:

Becker Muscular Dystrophy:
Similar to DMD in essential characteristics:
X-linked,
Calf hypertrophy, weakness is greatest proximally, and serum CK levels are high.
EMG and muscle histology are the same.
Two differences:
Age at onset (usually after age 12)
Rate of progression (still walking after age 20, often later).