Please help .

Just rescued an 8yr old male cockatiel about three weeks ago , he was in the smallest cage ever with nothing at all besides a food and water bowl . I know little to nothing about birds but I really want to help this guy out . He is terrified at just about everything ( treat holders , natural wood perches , toys , bird baths , any fruits or veggies ) except his food and water and I have to cut the millet treat in half or he will stay away from it the entire time . I’ve been sitting near his cage and talking to him and just trying to spend the day letting him know im not here to hurt him at all . Can anyone give me any tips on getting him comfortable with me , Id like to even be able to change his food and water without him being so scared . I also wish I knew what all his little chirps and sounds meant