Even if BM pointed his gun/brandished it at the first shooting scene (which could make him the initial aggressor) and they were then shot at by EN, once the Meyers left that scene and returned home, EN then became the assailant when he followed them home.
IANAL, but I think that once he was back at the house, BM would have been legally justified in shooting first when he saw the silver Audi approach the family home on the cul-de-sac, because it was apparent that he & his mother were being pursued by an armed assailant who had already shot at them at the first scene.
EN cannot claim self defense because, once the Meyers left the first scene, EN was no longer in any alleged immediate danger. There was no longer any imminent threat to his life. However, after EN fired those initial shots at the first scene and then pursued the Meyers' to their home, he was posing an ongoing imminent threat to BM & his mother.
I'm thinking the DA is looking at this as a case of self defense on BM's part (despite the yet-to-be-corroborated claim that BM may have brandished his weapon at the first shooting scene) due to the irrefutable fact that EN followed them home and shot at them again.