This a past the point of return, for this. The first item is it is easier to stop in the beginning than to cure later. All pups chase, period. It's a remedial problem now. I would not hunt the dog with other dogs, or people, until it's rectified. Some dogs will adjust on their own. The sequence would be keep running him into birds he can see, AND NOT CATCH! Eventually, because you have restraint and DO NOT KILL bumped birds, dogs fault or not, he will come to believe if it goes out wild, and does not get bagged, it's a poor result, for him and for you. This is the long way, unfortunately, there is not good way, other way is to reinforce whoa command, now. This is a little bit dangerous, go to far, at this point, might cow the dog down, destroy him confidence. A better way would do that in a chain gang situation, it helps to have other dogs who are dead on, reliable, who exhibit the right procedure, this is a learn and do exercise, easier done with a professional. In the future, all information you can instill in a pup, and I mean weeks old, including scent pointing, site pointing, and being tractable, are a whole lot easier. Here is why, a pup can only see a certain distance, because the sight cones in his eyes a developing. Because he relies on you, for guidance in a new surrounding, he stays with you. I use a mowed pasture, plant the quail, in vision. The dog or scents the bird, he then sees the bird, most all will point. I re-enforce point religiously, I walk around the pup, stroking and say Whoa, Steady, when satisfied I pick up the bird, with the dog still steady, then realize the pup, he will go to where the bird was, use his nose, only to discover it's gone! We do this repeatedly sometimes many times a day. Eventually, I flush the bird, dog remains steady, I release the dog, remember the sight issue, he can only see and judge distance a finite area. He will use his nose, most point again, an air washed bird. Some will pick them up, at that time it's a retrieving exercise, he has it, it's a big deal, little encouragement will crouch down and whistle, make a BIG deal. He saw a bird, pointed, pointed again, or retrieve a live bird. A little blank fire after he is released I this manner will put you a long way. He goes with you because you know where the birds are! He associates birds and you being equal in the process. Again this is a future event. Now you will need quit a bit of time to fix it now. Some will overcome this with repeated daily contact with birds. We have all heard the story that "that dog trained itself", it could be true, but what we can hope for is good genetics and some guidance. My experience is with English pointers, English setters, GSP's. Male setters are a little slower on uptake than female setters, in either case once they have the theory, they are ready to go. English pointers can take the training, then in the off season decide they have a better idea! A little training camp might be necessary. The GSP's seem to develop at the same speed. A male setter at that age can still be processing life! and can certainly be guided to a satisfactory conclusion. If you do it, it's satisfying, better for you and your hunting partner in the long haul, but you might be better off to hire a pro who has expertise, other dogs, equipment, and time!