If you do a search here on parrot bites in general and macaws in particular you'll come up with a plethora of threads on ways to handle that behavior. And you can also, as Walt (weco) suggested, find any number of videos online covering the subject.
There are 2 schools of thought at the polar ends of these approaches. On one end is the belief that one should only ever use positive reinforcement in parrot behavioral training. One of the most well-known and respected advocates of this approach is Barbara Heidenreich. She has several videos you may find extremely helpful. And the member most associated with that approach, should you do a search here, is MonicaMc. She has posted often, and quite eloquently, about this approach.
On the other side of the spectrum, among our members, are Mark (Birdmann666) and Richard (Henpecked). Both have worked extensively with rescues and have rehabbed birds with severe behavioral issues.
Doing a search on both approaches would be highly beneficial to you, I believe. It definitely was for me. You can learn a lot from either side of the debate even as you get a feel for both and see which way works best for you and your fid.
Personally, I tend to fall somewhere in the middle.
In your situation, given that he is relatively new to your household, my first step would be to work on your bond with Nemo. Bribery works great! Lol! You want him to start to feel like hanging out with you is fun! When you present your arm for step up, offer a treat just beyond reach so that stepping onto your arm leads to a tasty reward. (Try to do this part between meals so that he is hungry and motivated. Not advocating withholding food, here, just strategic timing.)
You want him to begin associating you with a good time. Then he will want to go to you. You'll be his preferred place.
You also want to work on target training. This will get him used to the idea that doing as you ask will always lead to good things.
Try not to rush things. The trust will build at his pace. Going too fast can damage the rapport you are building.
If he does bite, make sure not to allow yourself an animated reaction, whether physically or vocally. Some parrots find such displays amusing and will want to duplicate the behavior. Now, this isn't to say that you should just sit there and take it, either. I've never been a fan of that approach. Instead, calmly push into the bite instead of trying to pull away. (Counter-intuitive, I know, but pulling away often makes a bad bite worse as your parrot goes from delivering a message to holding on for dear life!) Then reach over with your other hand and take control of his beak. Just above and to either side of the point of his upper beak will grant you the leverage you need over the pressure he can bring to bear. Calmly turn the beak up and away from where he has clamped on until you manage to free yourself, all while telling him "No" (or "Gentle", or "Nice", or whatever word you want to use for your chill out catch phrase.) in a stern, but even, voice.
And make sure that you don't show him any hesitation or fear. Never let a parrot know that they can intimidate you. If they feel they can control you, they'll often work that angle.
Anyhow, once he's bonded to you, timeouts will become an effective disciplinary tool. Just turning my back on Bixby or Maya really bothers them. They'll want to avoid any behavior that makes you ignore them. (Keep the timeouts relatively short to make your point. Any longer than 15 minutes and you run the risk of their forgetting why they wound up on timeout in the first place.)
Sounds like things are progressing rather well with Nemo, though. It's good that you're spending so much more time with him than he was used to in his prior home... so long as that's the level of attention that you intend to continue lavishing upon him. But that kind of healthy and frequent interaction will make for a happy parrot and a healthy relationship between the two of you. Good luck, and I hope this helps!