baby crying at night

My LO is 18 weeks. He does not like to be rocked or anything to sleep anymore, so we have just had to lay him down in his crib. For naps he is great... he cries or fusses for at most 5 minutes usually. However, at night it is almost 30 minutes. We go in and soothe him at different intervals. Sometimes we have to pick him up and walk around with him if he is too wound up. Does anyone else have this problem? Any suggestions?