Rocking to sleep

So I've been trying to work on getting my baby to fall asleep independently. He's 18 weeks, and has always been a great night sleeper, but he won't take daytime naps unless he is rocked to sleep. I've tried laying him in his crib when he's drowsy but as soon as I do he is wide awake and cries. I've tried laying him down before he gets sleepy and he'll lay there contentedly but won't go to sleep. I always end up giving in and rocking him to sleep because if I don't he will stay awake until he's hungry and end up nursing to sleep. Any suggestions?