  We present a dynamical analysis of a quasar-host galaxy at $z\simeq 6$ (SDSS
J2310+1855) using a high-resolution ALMA observation of the [CII] emission
line. The observed rotation curve was fitted with mass models that considered
the gravitational contribution of a thick gas disc, a thick star-forming
stellar disc, and a central mass concentration, which is likely due to a
combination of a spheroidal component (i.e. a stellar bulge) and a supermassive
black hole (SMBH). The SMBH mass of $5\times 10^9\ \rm M_{\odot}$, previously
measured using the CIV and MgII emission lines, is not sufficient to explain
the high velocities in the central regions. Our dynamical model suggests the
presence of a stellar bulge with a mass of $\rm M_{bulge}\sim 10^{10}\ \rm
M_{\odot}$ in this object, when the Universe was younger than 1 Gyr. To finally
be located on the local $M_{\rm SMBH}-M_{\rm bulge}$ relation, the bulge mass
should increase by a factor of $\sim$40 from $z=6$ to 0, while the SMBH mass
should grow by a factor of 4 at most. This points towards asynchronous
galaxy-BH co-evolution. Imaging with the JWST will allow us to validate this
scenario.
