Hello again. On lead walking is fine - it's the off lead walking that's likely to do harm if he is afforded the opportunity to free hunt. So what you're doing is perfect. Just try to make sure all the time with him out is interactive. Playing with him, getting him to find the ball, making him sit and stay while you walk around him.
If you can get him at all interested in the ball, then try throwing in into a bush where he can't see it land. And then encourage him to go in and hunt for it. Show him it's really interesting and fun finding stuff. When he finds it lots of praise and encourage him to bring it back to you. He can do this while on a line (if you don't feel confident he won't bolt), or let him off to hunt for it as you can do this at close proximity to the bush.
Something others have done with some success is to feed all his meals during training. If you only raw feed this could be a bit more difficult. But if not - may be worth a try. He should then learn that all good comes from you, and especially if he's hungry, he's more likely to want to interact with you if you're the food source!
A word on the stop whistle - is try to make sure he is rewarded for stopping - otherwise it becomes a negative thing and he's unlikely to want to respond with much enthusiasm. Too often it's used to stop hunting or whatever. Reward him with food, or a retrieve (if he likes that), or even with being able to resume hunting. And don't try it if he's already bolting.
I appreciate it's going to be a very long road for you and this dog as he has learned that hunting for himself is the most rewarding thing. It's going to be an uphill struggle to try and change this. Not sure whether any of the above is helpful, but whatever - I hope you make progress with him as once you do, it will be hugely rewarding (for both him and you).
