You did not read my post - I took him off the seed - he came to me hooked on it - I have had him for about 3 weeks now. He doesn't take food from my hand.
Ok, so you've defined his interest and what the cause of his begging. You're way ahead of the game.
A bit of seed is absolutely safe for a cockatoo, so if you want to provide him with a some, given his age and emotional needs, do so...it wont kill him or hurt him even in the short term. Also, dont worry that he will define that he gets seed everytime he begs over the long-run. He is a weanling and should be treated as such...you'll have plenty of time to modify his dietary wishes, but only specific developmental periods in which you can impact his long term well being. Dont put too much pressure on yourself or the bird to do what is perfect....its an art and a science...focus on the "art" presently.
If you decide not to give him seed offer him portions of his staple diet and reassure him with affection. This will create an understanding that the staple diet is associated with the need for food, and also "comforting" the bird diminishing the likliehood that an unwanted, creative, concepts of negativity on the birds part.