I think the noun [***introvert***][1] or ***introversion*** is appropriate, as your friend may choose to keep his emotions unexpressed simply by nature, as a trait of his character.

I am also thinking of ***[temperate][2], [moderate][3]*** or simply ***quiet***.


  [1]: https://www.wordhippo.com/what-is/another-word-for/introvert.html
  [2]: https://www.merriam-webster.com/dictionary/temperate
  [3]: https://www.merriam-webster.com/dictionary/moderate