The Sage Advice Compendium 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.