20 month old would rather sleep in my bed rather then his cot. Any ideas or advice!!?

My son is 20 months old and for the past 2 months his been sleeping with me in my bed. Sometimes i will put him to sleep on my bed then put him into his cot when his gone to sleep, but then he'll wake up during the night wanting to come & sleep in my bed. At times i leave him in my bed because i get over tired, when i do leave him sleep in my bed he sleeps all night. Before he starting sleeping on my bed, he was sleeping all night in his cot. Any ideas or advice how to get him sleeping back into his cot? Sorry if it doesn't make sense.