What happens to Jan in the end of The Silver Sword?

The Silver Sword is a novel by Ian Serraillier, first published in 1956. The Silver Sword follows members of the Polish Balicki family who journey to reunite with each other after being seperated following the Nazi invasion of Poland.
Both the family patriarch, Joseph, and his three children are assisted by an orphaned pickpocket boy named Jan, who is adopted into the reunited Balicki family after a long trek with the three children to Switzerland.
Jan asking to be adopted by the Balickis and them granting his request is an incredibly special moment in The Silver Sword. Jan behaves rather selflessly in his journey to reunite the children to their parents and to keep his promise to Joseph, despite his own grief and psychological turmoil from the events of the war. With Joseph and Margrit's silver dagger, Jan is able to gain the trust of the Balicki children and begin the process of joining their family.
However, this process is not easy for Jan, as he has lived in isolation for so long that he appears to rely far more on his connection with animals than with humans. Eventually, his love and devotion to the Balicki children is shown to overwhelm his reliance on animals and fear of human connection. This is proven when he sadly leaves the dog, Ludwig, in order to protect the children in the storm before reaching Switzerland.

With a great effort of will he shed Ludwig from his mind and turned to his friends. (Serraillier 177)

In the end, Jan is able to balance his love for animals with his need to connect with humans and be a part of a family again. After being adopted by the Balicki family, he begins looking after a cat named Arlo and frequently works in helping treat sick animals.