You have a dog which is a cross of two breeds very prey driven. When locked into something, he will pay no attention. (I know I have a Belgian Malinois -- same problem). What I found helps is this. Calling come, baiting with treats won't work. If the dog is off leash in the yard when this happens, go up to him, snap his leash on, and tell him good dog or whatever. He needs first at that point praise that you have "seen" whatever it is he has seen, even if you don't. Then, using a GENTLE hand to turn his nose away, all the time telling him good dog and try and get him to focus on you. At that point, shove a treat under his nose and see if you can get him turned around and start walking with him away from whatever it is that has caught his attention. He will keep turning back, no doubt, but be persisent in a gentle manner, talking to him all the time in a normal voice.
A harness when walking would be good. I personally used a pinch but it must be used correctly and often did not work. So when walking and dog got fixated I did what I did above. The dog has stopped and is staring at something. Dog kept want to keep turning toward "prey" and I kept turning head around with a command to "look at me" or whatever you use and start to walk in the other direction at all times talking to your dog in a normal voice. And if dog will take a treat at this point use it then. You might trying giving him a command like "Ignore" while you are doing this having of course praised him first for finding prey.
I know this sounds kind of corny but after awhile it will work. I can now approach my dog when in prey mode and get her attention and she will turn to come and get the treat. At which time, i grab her collar and start walking away with her.
Unfortunately, this will take some time and effort, but I think it will work. My dog is ADD OCD (Attention Deficit Dog and Obsessive Compulsive Dog). It has definitely been a training nightmare. But we are coming along and hopefully you and your beautiful dog will to. Don't give up.