My 4 year old male chihuahua is extremely agressive, please read?

i have had my dog since he was 3 weeks old, too young for me to have him but the cirstances he was in he didnt need to be, my friend actually convinced his owner to give him to him because my chi as well as his parents were neglected in seperate cages and living in their own feces. my friend who took him couldnt keep him so i decided to take in the poor guy. i socialized him when he was young and took him everywhere, he is still to this day very healthy, up to date on all shots and medications.. well his issue is he is people agressive. he has bit my dad and brother and even me. i now live with a roommate and he almost bit her last night. he can sit, and come.. but he wont do it when new people are around, its like he flips a switch and is focused on nothing but getting to them, when they come in the door he attacks unless im right there calming him and then they pet him and hes okay with them, he will sit with them but if they go to move he will growl and snap at them. i dont trust him. when he barks i squirt him with a water bottle and say NO but as soon as i turn my back hes doing it again. ive had him tested extensively at the vet and he has no medical issues, he was put on prozac and is still the same.. im at a loss and not sure what to do. he wont let anyone get near me. hes so protective but as soon as we are out on a walk, the vet, the groomer, he is on his best behavior but he is scared, he nipped the woman who clips his nails. im not sure where to go from here.. ive taken him to training and behavior cles as well.. he cowered in the corner the whole time.. i do not want to lose him, i love him dearly but im scared he will bite the wrong person and i will be forced to put him to sleep. if i try to re-home him ill feel like ive given up. please help me.. what can i do?