How to get cockatiel's trust to scratch them?

my cockatiel gets close to me and lowers his head. but when he gets within range of my hand, he starts to bite. what exercises works to build the trust, or do i wait there patiently? i havnt been doing this much, but i grab a small piece of millet with my thumb and index and gently scratch him with it.i dont know if he likes it but its a bit safer for me from bites. advice?