  We study a disordered weakly-coupled superconductor around the Anderson
transition by solving numerically the Bogoliubov-de Gennes (BdG) equations in a
three dimensional lattice of size up to $20\times20\times20$ in the presence of
a random potential. The spatial average of the order parameter is moderately
enhanced as disorder approaches the transition but decreases sharply in the
insulating region. The spatial distribution of the order parameter is sensitive
to the disorder strength: for intermediate disorders below the transition, we
already observe a highly asymmetric distribution with an exponential tail.
Around the transition, it is well described by a log-normal distribution and a
parabolic singularity spectrum. These features are typical of a multifractal
measure. We determine quantitatively the critical disorder at which the
insulator transition occurs by an analysis of level statistics in the spectral
region that contributes to the formation of the order parameter. Interestingly,
spectral correlations at the transition are similar to those found in
non-interacting disordered systems at the Anderson transition. A percolation
analysis suggests that the loss of phase coherence may occur around the
critical disorder.
