In a comment to one of the other answers, you [said][1] that "he is a man with lovely feelings that he keeps private."

It seems to me that [**private**][2] would work well:

> [Merriam-Webster]

> **3 a** : withdrawn from company or observation  
> // a private retreat  
> **b** : not known or intended to be known publicly : SECRET  
> **c** : preferring to keep personal affairs to oneself : valuing privacy highly 

So:

> In general, he is private.

Or:

> In general, he is a private person.

I qualified this with *in general* because, as you said, he doesn't *always* keep things to himself, and not with everyone.

You could also say that it's his sense of *privacy* that keeps his emotions hidden from other people.

---

In terms of your example sentence, if you're looking for a quality that actively *enables* him to keep his emotions to himself, I would say:

> His ***self-composed*** **nature** enabled him to hide his strong emotions from others.

Merriam-Webster defines [*self-composed*][3] as:

> : having control over one's emotions : CALM


  [1]: https://english.stackexchange.com/questions/478942/a-word-for-hiding-strong-emotions/479008#comment1156282_478959
  [2]: https://www.merriam-webster.com/dictionary/private
  [3]: https://www.merriam-webster.com/dictionary/self-composed