  We present a multi-wavelength analysis, from the radio to the X-ray band, of
the redshift $z=6.44$ VIK J2318$-$31 radio-loud (RL) quasi stellar object
(QSO), one of the most distant currently known in this class. The work is based
on newly obtained (uGMRT, ATCA, Chandra) as well as archival (GNIRS and
X-Shooter) dedicated observations that have not been published yet. Based on
the observed X-ray and radio emission, its relativistic jets are likely young
and misaligned from our line of sight. Moreover, we can confirm, with
simultaneous observations, the presence of a turnover in the radio spectrum at
$\nu_{\rm peak} \sim 650$ MHz which is unlikely to be associated with
self-synchrotron absorption. From the NIR spectrum we derived the mass of the
central black hole, M$_{\rm BH}=8.1^{+6.8}_{-5.6} \times 10^8 {\rm M_{\odot}}$,
and the Eddington ratio, $\lambda_{\rm EDD} = 0.8^{+0.8}_{-0.6}$, using broad
emission lines as well as an accretion disc model fit to the continuum
emission. Given the high accretion rate, the presence of a
$\sim$8$\times$10$^8$ M$_\odot$ black hole at $z=6.44$ can be explained by a
seed black hole ($\sim$10$^{4}$ M$_\odot$) that formed at $z\sim25$, assuming a
radiative efficiency $\eta_{\rm d}\sim0.1$. However, by assuming $\eta_{\rm
d}\sim0.3$, as expected for jetted systems, the mass observed would challenge
current theoretical models of black hole formation.
