More specifically, how can Support be my MC's Symptom if Disbelief is his Problem? I don't under the context of Support here.

It makes a bit of difference if your MC is a change character or a steadfast character, but let's assume he's a change character for a moment.

In a story like this, the MC with THINK his problem comes from support, and he THINKS the solution to his problem is to oppose.  What he doesn't realize until late in the story is that he is truly driven by disbelief and only if he has faith will he have the possibility of resolving his personal issues.

For example, George hates it when his step-father tries to be supportive and responds by begin opposing every effort to engage him.  George thinks that as long as he plays naysayer he can keep himself from being hurt further.  As a START character he holds back when a bit of effort on his part could smooth out conflict, and only after time and a lot of resistance does George eventually get to the point where he can recognize that it is his own fundamental disbelief that anyone would want to be his father is at the heart of his fear of rejection and his dream that his biological father will return.  At a critical moment in his personal struggle, George chooses to have faith that there can be someone who cares about him enough to become the father he never dared to think he deserved...