One possibility is, that he is maybe a little over the top with the clingyness and so you are feeling smothered or that he is moving too fast for you. You can set the pace that is comfortable for you and you should do that if you think this may be the problem.

Some people will also find themselves in this kind of situation if they have a fear of intimacy or are emotionally unavailable on some level. They attract people with whom it would be difficult for them to connect with thereby, sabotaging the chance for a real relationship. It may last for a little while but usually drops off when the emotional connection starts to develop.

You may want to consider the possibility that there is something about him that reminds you of someone from your past -- a mannerism, look, attitude, etc., with whom you had a negative experience and you are now projecting into this scenario. Think about the first possibility for a bit as it is maybe the most likely.