How come Guru Nanak Dev Ji’s son abandoned Sikhi? What was the reason behind it?

Respected S. Jez Singh Ji;
Thank you for asking this question.
I am not aware of any references to it in Sri Guru Granth Sahib.
One could easily ask the question, how come Guru Nanak Sahib did not follow the religion of his parents?
It is an individual choice. Guru Nanak Sahib did not force his son to follow his religion.
Please remember that Sri Chand, Guru Nanak Sahib’s son, grew up in absence of Guru Nanak Sahib from home. While growing up Sri Chand must have been highly influenced by the local Pandits who, the Pandits, for their own interests, were against Guru Nanak Sahib’s philosophy.
Sri Chand was contestant to the Gurgaddi of Guru Nanak Sahib, but Guru Nanak Sahib, skipped him and named Bhai Lehnha ji as the second Sikhi Guru. This could not have gone well with Sri Chand and he started his own Panth, named UDAASEE.
Hope that explains at least one reason why Guru Nanak Sahib’s son did not follow Sikhi.