Can I say he wears a poker face?
Not in the situation you describe because a "poker face" is one that does not show emotion, that doesn't give away to the other players what the person is feeling or thinking.
The sort of description you want depends on whether the person is mainly just disappointed, or is more angry or upset, and other factors. Possible ways to describe the loser's look are: dejected, bitter, beaten, defeated, crushed, long-faced, glum, crestfallen, down at the mouth, despairing, dismal, resentful etc.
not a teacher
Student or Learner