Need help introducing cat and kitten?

We have just saved a female kitten who is now 15 weeks old. We have tried introducing her slowly to my 3 year old male cat who is not used to having another cat in the house and I'm not sure if things are going well or not. After 3 weeks they will come up and sniff noses and can sit in the same room but now my male cat has started chasing the little girl and strikes with his front paws and bails her up in a corner. She rolls over in submission? but he still keeps it up with both ears back. Eventually I break it up with a squirt of water and he walks off in a huff clearly unhappy only to do it again next time. Does this sound like what usually happens when things are going well or is it a step backwards? I'm thinking of using a feliway diffuser but not sure if it will help? I understand that cats need time to get along but I'm just concerned about the little one getting hurt or getting too afraid.