@optimisticStar It is wonderful that you got to meet your young bird, too bad you have to wait so long to get him home. As a little bird he will be more respective of touch/petting. I do not think should be expected when he is an adult bird. It isn't a normal behaviour for adults of his species so "training" them to tolerate it is probably not the best idea. Teaching him a trick--laying in a hand for a treat-- this may work. I think you will have better success building trust if you accept his nature.
I don't know if you have experience with cats, but this might help as an comparison. If you pet a cat on the head of back they are happy as a pea in a pod. If you pet a cats stomach they will usually nip or bite. This isn't because the cat is a bad cat, or hasn't been trained to like tummy rubs.. It is because it is overwhelmingly stimulating to the cat's sensitive tummy. Dogs on the other hand love a tummy rub, because this is a natural behaviour in dogs. This is a way they can show their trust and position in their pack.
Encouraging or promoting a non allopreening bird to accept scritches can become very stimulating and overwhelming to the bird. I think it will be a much stronger and healthier relationship to respect the bird's boundary this way. Imagine your friend touching the most ticklish spot on your body---you might become numb to it over time.
My plummie loves being on me. He runs up and down my sleeves, sleeps on my shoulder and hops on my lap. I don't touch his head. I don't pet his back. -He loves me all the same. He is so excited to see me in the morning and he sings all the time. Touch is really not required to have a super awesome bird friend.