as far as i can tell they pretty much alluded to what happens....
he takes a ton of those booster shots and eventually destroys his body with them
like his eyes rot/stop working, his leg stops working....
he feels he can no longer be a good partner to Eve so he sewer slides
Eve finds him, and presumably the on-site team saves him in time, OR Eve brings him back to life
Eve erases certain memories of Adam's, and almost all of his own memories
sometime thereafter the series starts

adam saying he wasnt gonna die makes me thinks mth rlly bad appened to him during this flasback