It sounds like your partner is [***reserved***][1]. From Oxford Dictionaries:

 > Slow to reveal emotion or opinions

This fits your description of your partner as someone who generally keeps his emotions hidden, but who does share them with you (though, I'm guessing, only after a gradual process of getting to know you). Note that the term *reserved* doesn't imply a lack of emotions or opinions. In fact, the [definition from Collins Dictionar][2]y is

> Someone who . . . keeps their feelings hidden

which (in addition to sounding very much like your description) strongly implies that there *are* emotions to be hidden. However, a person who is reserved could be *mistaken* as unemotional by people who don't see beneath the surface calm.

For your example sentence, you need the noun form.<sup>1</sup> From Cambridge Dictionaries:

> [**reserve**][3] *noun* (SHY BEHAVIOUR)  
★ [ U ]  tending to keep your feelings or thoughts private rather than showing them

So you could say

> His **reserve** enabled him to hide his strong emotions from others.

You could also add an adjective to make the sentence more pointed, as in *his natural reserve* or *his habitual reserve*. Similarly, if you wanted to stick with the adjective, you could rephrase slightly: 

> His **reserved (nature/demeanor/etc.)** enabled him to hide his strong emotions from others

Or

> Because he was (always/naturally/habitually) **reserved**, he was able to hide his strong emotions from others


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<sup>1</sup> <sub>Of course there is also the noun form *reservation*, but that generally isn't used with this sense of *reserve* and *reserved*: When a table is reserved, someone has a reservation; but when a person is reserved, that person only has reserve.</sub>

  [1]: https://en.oxforddictionaries.com/definition/reserved
  [2]: https://www.collinsdictionary.com/us/dictionary/english/reserved
  [3]: https://dictionary.cambridge.org/us/dictionary/english/reserve?q=Reserve