The subject of the email from Barry S. first reads "we need you", but after opening it reads "RE: No subject"
In "Unbreakable", David Dunn survives a train crash and at one point he uses his power and senses an abusive mother who mistreats her child. And in "Split" it is revealed that Kevin's father left him on a train and he was forced to stay with his abusive mother after. Many have speculated that the same train-crash David Dunn survived killed Kevin's father and that the abusive mother David senses is Kevin's own mother. It is possible, as director M. Night Shyamalan has mentioned that Kevin is supposed to be in his mid-20's in "Split", and has been working at the Zoo for ten years... so it is conceivable he got his job in his mid-to-late teens and has been working there ever since.