The [Sage Advice Compendium][1] includes a ruling on the DM (not the player) deciding what is summoned with a Summon X spell, and also includes a ruling on dispelling such effects once created:

> Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend.
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> In contrast, a spell like *conjure woodland beings* has a non-instantaneous duration, which means its creations can be ended by *dispel magic* and they temporarily disappear within an *antimagic field*.

Notice here (backed up by Mearls as asked by @Christopher) that they are saying the creations of a Summon X spell can be ended by *dispel magic* or temporarily vanish inside *antimagic field* (because they ARE magic) **individually**, but that one cast of *dispel magic* does not destroy them ALL. Targeting the caster with *dispel magic* would do nothing because the spell is not affecting him as a target; he is simply channeling the spell. 

Breaking concentration would be the fastest way to deal with multiple summoned creatures since *dispel magic* doesn't have an AoE. The aforementioned *antimagic field* would also be effective if the caster was caught inside, rendering his concentration spell non-functioning in its entirety until he stepped outside of the antimagic zone again, in which case the creatures would reappear.

 [1]: https://media.wizards.com/2019/dnd/downloads/SA-Compendium.pdf#page=14