From the Sage Advice Compendium including rules on the DM (not the player) deciding what is summoned with a Summon X spell, and 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.
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 Anti magic (because they ARE magic) **individually**, but that one cast of dispel magic does not destroy them ALL. Targeting the caster with Dispel 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 unless Dispel Magic had an AoE. The aforementioned Anti Magic 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 Anti Magic zone again. In which case the creatures would reappear.