He has a serious-looking face perhaps due to the fact that he is at work and is trying to be professional.
That being said, so far, he hasn’t shown any significant signs that he may have feelings for you or is attracted to you.
You have made eye contact with him a few times and he has given you little smiles.
However, this could have been the same response he would give to anyone that he makes eye contact with in an attempt to be polite.
What you can do is open up a conversation with him the next time that you see him.
Being that this is a guy that is at your work, it would be easy to start a conversation with him.
You could just reference something about work as your opener.
Once you actually open a conversation with him, it will be easier to determine whether he is attracted to you or has feelings for you based on how he interacts with you from then on.