A. it could be due to lack of feathers on that side, maybe he broke or lost a few feathers on this side and this may affect his ability to fly. Was he always like that? if so his wing's feathers might have been clipped by the previous owner.
it could also be a medical condition. Skin tumors on the wings (sometimes on the inner side) can be quiet common especially in older parakeets. Some internal diseases can also affect the ability to fly (neurological diseases and others).
If it carries on and doesn't get better try and take him to an avian vet.
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