  SN 2008es is one of the rare cases of a Type II superluminous supernova
(SLSN) showing no relatively narrow features in its early-time spectra, and
therefore its powering mechanism is under debate between circumstellar
interaction (CSI) and magnetar spin-down. Late-time data are required for
better constraints. We present optical and near-infrared (NIR) photometry
obtained from Gemini, Keck, and Palomar Observatories from 192 to 554 days
after explosion. Only broad H$\alpha$ emission is detected in a Gemini spectrum
at 288 days. The line profile exhibits red-wing attenuation relative to the
early-time spectrum. In addition to the cooling SN photosphere, a NIR excess
with blackbody temperature $\sim1500$ K and radius $\sim10^{16}$ cm is
observed. This evidence supports dust condensation in the cool dense shell
being responsible for the spectral evolution and NIR excess. We favour CSI,
with $\sim2$--3 $\textrm{M}_\odot$ of circumstellar material (CSM) and
$\sim$10--20 $\textrm{M}_\odot$ of ejecta, as the powering mechanism, which
still dominates at our late-time epochs. Both models of uniform density and
steady wind fit the data equally well, with an effective CSM radius $\sim
10^{15}$ cm, supporting the efficient conversion of shock energy to radiation
by CSI. A low amount ($\lesssim 0.4$ $\textrm{M}_\odot$) of $^{56}$Ni is
possible but cannot be verified yet, since the light curve is dominated by CSI.
The magnetar spin-down powering mechanism cannot be ruled out, but is less
favoured because it overpredicts the late-time fluxes and may be inconsistent
with the presence of dust.
