Hi Sai,
have only just seen this post, sorry.
I used to be a supervisor in a day nursery baby room so I have been on the other end of this dilema!
I think you are probably right when you say you think only leaving him for half an hour might be part of the problem. In my experience, an hour was ok for the initial visit but several longer visits were often needed in the lead up to actually starting. I usually found with more clingy children that it could take up to an hour for them to get past the really tearful on-off crying stage and then they would start to feel more secure and begin to explore. Half an hour is not really long enough for them to start to build up an attatchment to anyone.
I don't know what your work situation is but if at all possible I would suggest an extended settling in process but for longer periods, maybe a couple of hours. Have a chat withn the nursery, You could maybe have an agreement that you will leave him for two hours but if he really is getting hysterical and is not showing signs of calming down they are to call you.
Be prepared for him to cry but it might feel a bit easier if you know that if he really won't calm down they will call you. Another thing is, the more times you leave him, the quicker he will learn that mummy always comes back! He might just suprise you and be playing happily when you come two hours later.
(tip-try to peep in the window without him seeing you first coz he may well cry with overwhelming joy when he sees you again)
Hope this is some help, feel free to ask if you have more questions