Honestly, I'm a bit confused. Was this the work of the shadow figure from the previous chapter? If so, why leave the kid alive and not finish the job? Seems pretty inefficient to poison the kid and leave the kid to die, since that leave things to chance such as the kid surviving on his own and possibly gaining a resistance to the poison and/or someone helping him.
Honestly I think this was either just a coincidence and the shadow figure haven't made their move yet, or this was the shadow figure's doing, but they never intended to kill the kid, but instead they intend to go after the Demon Lord and are using the kid to test the water and see how much the Demon Lord cares for the kid in order to enact their plan.