This is not an exotic problem. This happens with a lot of dogs that are not trained right from the start. Bernhard FLinks method for young dogs is great. However it will not always work with the older dogs. You sometimes run into dominant issues. One of two things though. Stop playing ball with him so he does not have to out, unitl you are ready to deal with the probelm, as you are creating a bad habit, or address the issue now . Two months IF the dog is not handler aggressive, in my opinion is not to early to start complusion on exersises. The clearer you make it for the dog, the better the bond will become.
To start off, you need a prong. I have flanked dogs that do not like to out like Benhard does, and it does work a lot of the time. Somethimes the dogs will take a cheap shot at your hand, but just be quick, they really do not like this at all. You can hold his collar, pinch his flank, and say out; holding the collar gives you more control and thus he cannot snap at you to easily. The trick is if you can, do not let him see your hand pinch him, but this is not always possiable. Once he outs kick the prey toy, and tell him packin or whatever word you use.
Another thing is ,tell him out, and give him three of four good quick hard jerks, with the tight prong, and then if he outs kick the ball or whatever away and let him chase it and get it.
Remember that you must be carefull doing any kind of complusion as this dog has come to you as an adult; there could be things that you do not know about him, like he mauled his last handler for the same reason. I would back tie him to be safe, or two leash him.
You can also go to the electric, but again you have to make sure you do not turn him on and create more problems.
You can also, use a sharp-prong. If he is a problem to out, one or two quick jerks with a sharp prong, and he should comply. But again, he could get nasty, so do not do these things without precautions.
It is my feeling that this is a training issue, that tricks will not work for him. As someone else said, he will learn the two ball thing, and not out when he has had enough. He needs to understand that when you say out, he must let go or you will punish him. It sounds like you have a good strong dog, so this is the only way to teach him. After you get him to out, you can make the dog out longer and longer so that he does not rebite everytime right away. I would recommend that you buy Bernhard Flinks tape that deals with Drive FOcus and Grip. While there is not much info on the out, it will open your eyes to the light in which the toy should be used. I learned alot.
One thing is for sure, if you do not get a handle on this soon, it will most likley be a problem forever. As the dog ages, the more trouble the out will be to train. You cannot fix this, if this dog is as strong as you say with flat-collars, or treats.