major separation anxiety is very common at that age.he's just confirming that he thinks you are the most important person in his life and doesn't want you to leave him. forcing separation definetly makes it worse and destroys his trust. can you stay in his room before he goes to sleep in the cot? rub/pat his back. pick him up and cuddle him if he gets upset. gradually he will grow out of it and you slowly move from holding him, to sitting by the cot, to sitting nearer to the doorway, then out the door! LOL.
goodluck