Hi Denise
Sorry my ideas didnt work for you.Does it matter if he does not retrieve?.
Do you need it for dog training,not being facetious but I have not been to training classes (only Gundog) when you certainly need a retrieve.My old dog Murphy a Worker( sadly no longer with us) ,would not retrieve as a puppy .he would sit and stay while I threw the dummy and mark it every time,when I sent him for it he would go off smartly straight to it but would not pick it.As I go beating and not picking up I didnt worry about it. Then out on a shoot when he was about 2 years old, beating in a wood he returned to me with a pheasant in his mouth which he had seen fall after being shot,after that he picked any sort of bird and his retrieve was no longer a problem.So it does happen that they are late retrievers.Perhaps it would be best if you left it for a while and go back to it at a later date.
Good Luck