How can I balance the redox reaction of sulfur dioxide with oxygen?
WELL I think you put something erroneous in your reduction formula. 

You will never get SO3 out of just O2 and water. Instead of SO3 

The reduction formla will be 4e- + O2 + 4H+ --> 2 H2O  

Multiply oxidation formula by 2 and add it to new reduction formula and you got your answer