Is there a term to describe an inability to express emotion or feelings? I am writing a novel and was wondering if there was a specific term for someone unable to express his own emotions or feelings. The catch here is, this doesn't mean he doesn't know what anger or love is for example, but just can't express it through his own body language. 

The main idea is someone like a method actor for example might have so much experience taking on a certain role and become so good at adopting other personalities and portraying them, that he has forgotten how to be himself. While he might be a expert at taking on the role of a scared child for example, when confronted with something fear inducing out of character, he simply can't express it whether verbally or physically.

Another example might be someone unable to express happiness, but when given dolls or sock puppets, he can very easily show happiness and mood through the puppet/doll. The only issue is expressing this through his own body.

If any of you have seen the TV show Hereos, there is a character called Sylar who has the power to steal other peoples abilities. This is all fine and dandy, but once he steals the ability to shape shift, he starts to lose a sense of who he is. While he can transform and impersonate other people flawlessly, each time he does so he seemed to lose a sense of who he was originally. Essentially, he slowly starts to forget how to be himself. This seems to be the best example I can think of that I've seen on TV.

In essence, I'm looking for a better way to describe what I've listed above in a way other than just 'someone who has trouble expressing emotion'.
It is alexithymia if you're looking for a diagnostic term. It is not a case of "you either have it or you don't—alexithymia is a continuum.

There even exists a scale, which is a professional scale so to get it you need to pay for it and be a researcher. It's called Toronto Alexithymia Scale (TAS-20).