Does a doppelganger's Change Shape special quality create the clothes of the creature the doppelganger changes into? If not, can I assume that after the transformation the doppelganger will still be wearing the same clothes he had been before?
Doppelgangers have a version of the Change Shape special ability. The linked rules specify what this ability lets you change, and creating new equipment isn't one of the possibilities.
The Change Shape ability originally made no mention of items at all, which left its behavior somewhat ambiguous, but the official errata to the Monster Manual (available here) added the following text to the ability:
Any gear worn or carried by the creature that can’t be worn or carried in its new form instead falls to the ground in its space. If the creature changes size, any gear it wears or carries that can be worn or carried in its new form changes size to match the new size. (Nonhumanoid-shaped creatures can’t wear armor designed for humanoid-shaped creatures, and viceversa.) Gear returns to normal size if dropped.
So no, a Doppelganger cannot create new clothes with its Change Shape ability. It will either continue wearing the same clothes as before (if they fit the new form), or its clothes will fall to the ground (if they don't fit the new form).