Good job in initiating conversation.
If he is now teasing you less than a week since you both first talked, he may be showing tentative signs of attraction.
He may be comfortable joking with you and teasing you so soon because he may have realized through your work-related conversations that the both of you have some similarities in personality and even temperament.
However, this may not be something that he uniquely experiences with you.
There may be a few other people at work that he has this kind of connection with.
It would be to your advantage to observe his behavior when he is interacting with co workers.
If there are some that he teases, this would indicate that your experience with him is not unique.
If he doesn’t tease other co workers, then again, he may be showing tentative signs of attraction.
He is still going to have a serious face at work at times.
That’s just him in work mode.
Try not to take it personally.
It is actually not directed at you.
He probably has no idea that you are as intimidated by his serious face as you are.
So far, you can read his actions by understanding that you have taken the first steps in opening him up and allowing him to feel more motivated to start conversations with you.
Last week, he actually initiated conversation with you.
This is a good sign.
It will still take some time for him to feel entirely at ease with you to the point where he suggests going out to lunch or meeting after work, but this is a point in time that may be possible.
This of course is predicated on him not already being taken.
It would be to your advantage to find out whether he is single or not as soon as you can.
This way, you don’t spend the next several weeks interacting with him and beginning to become more and more hopeful for romance, only to find out that it was all for naught because he is taken and emotionally unavailable to you.