This takes place in the Taiga Level
Going for the Forest Child Achievement should get you this Achievement as well
The Forest child achievement in Metro Exodus worth 63 pointsComplete the TAIGA level without attacking anyone or getting noticed.
If your not gonna be going for Forest Child as well the guide below should help
Alyosha's fate is connected with our attitude towards the locals. Avoid killing the enemies - spend the majority of the chapter on sneaking past human enemies. Going through settlements during the night is a good idea. Luckily for you, the majority of the enemies are carrying a crossbow - you will have plenty of time to run towards the opponents and stun them. If you come across prisoners, always rescue them. Thanks to that, Olga will appear at the end of the chapter - he will let you and Alyosha go in peace. You will also unlock the Alyosha achievement.
Otherwise, Alyosha will end up in a wheelchair.