Biting and screeching cockatiel

I have an approximately 2 y\o cockatiel. Originally I bought him from a pet store for my mom. According to the store he or she, no idea which, had been hand raised and fed. They said the bird was only a few months old. There were 2, and the one I bought was friendly and allowed myself and my daughter to handle him. Since bringing him home, my mom has always cleaned the cage and changed his food and water daily. When we first got him, she also taught him to talk. In the first few months of having him, he was skittish, but eventually would come on my finger, and allowed me to scratch his head. He never allowed my mom to scratch him, but would go on her finger. Also in the first few months he broke a few blood feathers. I did some reading, and since I'm a nurse, my mom held him wrapped in a towel, and I used hemostats to remove broken feather with bulb intact, so he wouldn't bleed to death. We've had to do this maybe 4 times in 2 yrs. Problem is, he doesn't like my mom, he's attached himself to me. He screeches when I'm not around, and when I come home, and whenever he wants my attention. The other problem is with my mom, he will go to her, and even sit on her shoulder etc,but all of a sudden, he will bite her, really hard, on hand, in face, wherever he can reach. For no reason, she is very nice to him. I on the other hand do get aggravated with bird, when he won't be quiet, mostly. But he doesn't bite me, I can do almost anything to him, I don't mean hurt him, but I can grasp his beak, stroke his whole body, kiss him, even hold his body with my hand around him, and he rarely if ever bites me. I'm trying to stop the screeching, I work, my mom is home with him all day, every day. Also, don't know how to stop him biting my mom for no reason. I want to keep him, but this is making it very difficult. Jane