Converting prefix to infix

With[{S = Infix[S[##]] &}, S[a, S[b, c]] ]

a ~S~ (b ~S~ c)

Also

S[a, S[b, c]] /. S -> (Infix[S[##]] &)

a ~S~ (b ~S~ c)

You could just give S a format:

MakeBoxes[S[a_, b_], StandardForm] ^:= MakeBoxes[Infix[S[a, b]], StandardForm]

Then:

S[a, S[b, c]]

a ~S~ (b ~S~ c)