Maybe wait a day or too.. you don't want to scare him off or seem too clingy. But if he IMs you.. go right ahead and message him back. But be careful, bcause IM can be suicide for a relationship. Sometimes people only talk on IM becuase they feel more confortable and funny and what not but then when it comes time to be with each other in person, it might seem awkward and weird because you don't know how to act.