On values of isotropic quadratic forms

Let $K$ be a locally compact non-discrete field of characteristic $p>2$ and $Q$ be a non-degenerate isotropic binary quadratic form with coefficients in $K$. We obtain asymptotic estimates for the number of solutions in the two-fold product of a discrete subring inside $K$, of the inequalities of the form $\mathrm{\mid }Q(x,y)\mathrm{\mid }<\delta$ for some $\delta >0$, where $\mathrm{\mid }\cdot \mathrm{\mid }$ is an ultrametric absolute value on $K$. The estimates are obtained in terms of continued fraction expansions of the coefficients of the quadratic form $Q$.