From the Weinreich dictionary:
son -- zundl (zundeleh) or zindl (zindeleh) -- both are kosher
sun -- nothing is offered for the diminutive
Personally, i'm sure the diminutive of sun is also zindl (zindeleh)-- i see no reasons why it shouldn't follow the common convention.
Also, zinehnyu was a popular diminutive in Russia and Ukraine. -nyu was added to create a diminutive, e.g. Moishe -- Moishenyu.