Aggressive cockatiel help?

I recently bought a male cockatiel from a pet shop, and he is quite aggressive. He is in a large cage and at first he would hiss and sway as soon as i walked into the room, and scream and fly (or hop or whatever, his wings are clipped) around the cage when i put my hand into the cage to change his food and water. Iv had him for 3 weeks and he has gotten a bit better, i can sit next to the cage and whistle at him and he usually whistles back, and when i change his food and water he will go the furthest perch and just watch me, and if i move slowly i can get my hand about 15cm from him but he moves away if i get any closer. Do i just keep going slowly or is there anything else i can do to gain his trust? Like any particular food that cockatiels really like that he might be willing to eat from my hand or something. Also, he has a block of cockatiel seed in his cage, plus a container or loose seed, and 2 or 3 millet sprays at a time, but he only seems to eat the millet sprays, there arent any marks in the block and he might have been eating some loose seed as there is less than there was but i havnt seen him eating it or if hes just spilling it, he doesnt seem to be losing any weight. Are millet sprays ok as the main food source? He is kept in a cage next to my budgie who is quite tame, and they whistle to each other regularly so dont think hes lonely. Can i feed him fruit and veggies? Are there any that are bad for him? Thanks in advance :)