  We calculate the magnetization of the helical metallic surface states of a
topological insulator. We account for the presence of a small sub-dominant
Schr{\"o}dinger piece in the Hamiltonian in addition to the dominant Dirac
contribution. This breaks particle-hole symmetry. The cross-section of the
upper Dirac cone narrows while that of the lower cone broadens. The sawtooth
pattern seen in the magnetization of the pure Dirac limit as a function of
chemical potential ($\mu$) is shifted; but, the quantization of the Hall
plateaus remains half integral. This is verified by taking the derivative of
the magnetization with respect to $\mu$. We compare our results with those when
the non-relativistic piece dominates over the relativistic contribution and the
quantization is integral. Analytic results for the magnetic oscillations are
obtained where we include a first order correction in the ratio of
non-relativistic to relativistic magnetic energy scales. Our fully quantum
mechanical derivations confirm the expectation of semiclassical theory except
for a small correction to the expected phase. There is a change in the overall
amplitude of the magnetic oscillations. The Dingle and temperature factors are
modified.
