I would agree with most of the above, but would not recommend a clip at this time. Larger birds are less likely to take flight inside as it is, they're heavier so it takes more effort to fly and there's usually just not as much room for them to fly in a typical home. I feel that the stress of putting him in a carrier to have the wings clipped would far outweigh any potential benefit, and I certainly wouldn't recommend clipping his wings on your own... you're likely to either give him an uneven clip, clip too much, or clip a blood feather (worst case).
I would say sharing food from your plate (unseasoned food! Grains and veggies, maybe fish, not red meat) would be great. Eating is a social activity for flock animals and if you eat the same meal together, or even different foods but at the same time, he'll see you as a flock member. Just be careful you don't turn him into a junk food junkie by taking this advice too far and sharing all of your food with him.
Training is a great starter activity too. Check out youtube videos on target training. I haven't seen this particular video, but BirdTricks is a really commonly recommended channel and they came up when I searched target training, it's probably worth a watch:
When training you want to use high value treats that aren't part of your birds regular diet, and which are very small and quick to eat. Common training treats include pine nuts, safflower/sunflower seeds, almond slivers, crushed pieces of walnut, etc. Experiment, see what he likes!
And above all, work at his pace. He's been through a lot. Keep an eye on his body language and the moment he seems uncomfortable, freeze and take a step back. You want to communicate to him with your own body language that you care about his comfort. There is NOTHING to gain by pushing his boundaries. Especially given that you're working with a cockatoo.
If he's your first cockatoo and you're not super familiar with cockatoo body language, you should probably prepare yourself to be bit. Birds almost always warn us before a bite, but the signs can be very subtle, and my experience with cockatoos is that sometimes bites can seem truly random. But if you work at his pace, respect his boundaries, and take a few steps back when he goes into those crazy excited cockatoo phases (lol) you'll probably avoid most bites.
Thanks so much for giving him a loving home, I hope it works out
