napping

My baby is 5 month old and since birth he will only nap in his rocker. However, when I go back to work in may, he will be going to nursery and they take naps in cots there so I'm just wondering how I can get my baby to take naps in his cot? He's fine on a night, he sleeps straight through in his cot but when I've tried making him nap in there through the day he wouldn't.