Identifying Lewis acid What is the Lewis acid on the reactants side?

$$\ce{HNO2(aq) + HPO4^{2-}(aq) &lt;=&gt; NO2-(aq) + H2PO4-(aq)}$$

The book says it is the $\ce{H+}$ only, not the whole molecule of the nitrous acid, how so?
What we have, in the Lewis theory, is an acid-base displacement reaction.  The Lewis acid, $\ce{H^+}$, is initially combined with one Lewis base, $\ce{NO2^-}$.  Then another Lewis base, $\ce{HPO4^{2-}}$, takes the $\ce{H^+}$ away displacing the $\ce{NO2^-}$.

Similarly when $\ce{HCl}$ is dissolved in water and dissociates, the water is displacing the chloride ion.  The Lewis acid involved in this displacement is again $\ce{H^+}$.