Lovely picture of Oscar Mandy - sorry to hear he has decided not to listen to your whistle. They do need constant reminders and I will call Maggie and Elsa back to the whistle at least once on every walk even if I don't need to.
What I would do in your situation is go back to basics. Firstly get something high value you can use only when you use the whistle ie a very tasty treat, one of his favourites or if he is not interested in food on a walk (Maggie isn't) one of his favourite toys will do - like his hedgehog. Once you have decided which toy take it away from him and only give it to him when he comes to the whistle so that he associates the whistle with a play with his favourite toy. Firstly do it in your back garden or even in the house only. When he comes give him the treat or a fun play with his toy - make the game really exciting for him. Then tell him enough and take the toy away this way you will make the toy even higher value to him and he will want it all the more. Once he is coming first time to the whistle every time at home then you can start practicing when you are out. Personally I wouldn't call him back to the whistle when you are out unless you are sure he will come to start with. If he is looking very busy hunting for instance there is a high chance he won't come and you will set yourself up for a fail. If he ever doesn't come after you have called him then either go and get him and not treat him or if that's impossible wait until he does come back and then ignore him. I have been told to only call once otherwise you teach them that calling two or three times is ok before they come back but sometimes I'm not sure she has heard so I do give her a couple of chances. I think he will progress quickly because he already knows what the whistle is - he just needs reminding why he needs to listen.
They ball is my key with Maggie - she will comeback from anywhere anytime now because of a slim chance of getting a game with the ball. She doesn't get it every time I call her now, but she doesn't know when so she comes back anyway. Elsa is more food orientated and will do just about anything for a bit of cheese. She is not as predictable as Maggie yet but she is still young.
Oh yes that's the other thing - if you choose a toy as your treat for recall make sure it's something that is easy to carry on walks with you - preferably without him knowing you have it.
Sorry gone on a bit - hope all that make sense.
