how do I get my nanday to stop bitting when I go to hold him?
slowly work up to it. He has to trust you before you can hold him. Try getting him off his cage or to a place where he feels safe and that you can work with him, perhaps a stand. Then work on step up and step down and simple excercises. Praise him when he does a good job. After a while of working with him, he'll understand that your ok and not going to hurt him and perhaps he'll let you cuddle him. Just remember, however, that if you try and pet him in a spot that he does not want to be touched, he'll respond with a nip or bite. His age and his circumstances all have alot to do with how well he progresses. If he has been re-homed and people ignored him in his last home it may be more difficult that if he is younger and yours is his first home. Oh, and don't forget that establishing a routine is a very good thing for a Nanday (or any bird), so that they know what to expect. For example my Nanday, Ozzy, knows when it's dinner time when we all sit down at the table which is not too far from his cage. When we start to eat he'll start eating his fresh veggies that we give him for "dinner".
I have a different suggestion. I am the owner of an eight year old female Nanday conure (Screech) and an African Grey. I've noticed my Nanday likes to step up by using first her beak to balance and then with her feet to actually step up. When I get bitten is when she uses her beak to steady herself (as she expresses her displeasure with me)while stepping up. If this is when you are being bitten, try this suggestion. Since Nandays are so smart, they are fairly easy to train. I am training mine to step up onto my finger using only her feet, no beak. When she puts her beak out, I tell her to use her feet only and I try to touch her feet while I am saying "feet". I say "step up" firmly and touch her feet and usually she steps right up without using her beak. If she still puts her beak out,I move my finger away as I tell her I will come back in a few minutes and we will try feet again. I use a lot of praise when she does it the way I want, which has become quite consistent! It is fairly imperative I not get bitten as I am allergic or sensitive to her beak bacteria and get infected at almost every bite. (This is Screech specific, my other bird's bite does not have the same effect on me.)