Telling someone you're interested but not ready for a commitment is a tricky thing to do. While you may have genuine feelings for this person, entering into a relationship is something you know you cannot handle at this time. There are a few effective ways to express interest in someone without committing.
Be honest about your interest. If you are interested in getting to know this person and eventually being in a relationship, be open about your feelings for him. Being transparent about this will help him to understand that you are genuinely interested in him, but you are simply not in the right place in your life at this time for a relationship.
Explain why a relationship would be too much to handle right now. Perhaps you are overwhelmed with assignments in graduate school, or you are dealing with a difficult illness in the family. You may be struggling to make ends meet financially and do not want to enter into a relationship until you are more stable. Whatever the case, be open and honest about the challenges you are facing.
Offer up a timetable. If you cannot be in a relationship now, discuss a timetable for a point in the near future when a relationship would be something you might be able to handle. If the end of graduate school will bring you freedom and time, explain that. If your upcoming promotion will put you in a better place financially, be upfront about that.
Keep communication open. If the person you are interested in decides he can wait until you are ready for a relationship, make sure to keep communication open during this pivotal time. You want to make sure he understands that you see yourself being available for a relationship soon and that you are working toward a place where a relationship will be possible. He will appreciate your openness and understand that you are not playing games.
Be prepared to let go. If life is genuinely stopping you from being in a relationship right now, the person you are interested in may not understand and decide not to give you a chance. If this is the case, stand your ground -- a person who does not understand why a relationship is not doable for you will likely not support you in more of life's obstacles. Recognize this as a sign that you will eventually find the right person for you.