This is Dr. David
he should see his cancer doctor again.
and get blood testing to check for low magnesium levels.
low magnesium levels or low potassium blood levels can often cause pain sin the muscles and can be lowered by chemotherapy
so these blood levels have to be checked.
also he might benefit from nerve pain medications like neurontin (gabapentin) or lyrica (pregabalin) which could help his pain.