We had the same problem. Some things you can try.
First, don't let it sit and go bad. Put it down, remove it if not eaten. I know cats like to nibble so down a few hours is OK, but not all day.
Try only very, very tiny bits at first, not a whole meal. Put it into small drops, sort of kibble shaped, that he can easily pick up.
Try a flavour he is already eating in dry food.
Mix a small bit in with the dry. Or, put a small bit in the dish beside the dry. Or put a small bit on a separate plate beside the dry. Decrease the dry while you try these but give it back later if it doesn't work. Some people report success with the small bit on the plate beside. You throw a lot out for maybe a couple of weeks but it seems they get used to having it beside them and eventually take a bite. Maybe.
Honestly we had limited succes with all of these with our cat who needed the wet food the most. What marked success for us was an off chance comment from the Vet that she loved the Recovery food they fed her during a stay there. A stay, by the way, prompted by very bad constipation which the dry food contributed to. We bought several cans of expensive Recovery wet food and that was how we transitioned her to store bought canned. It was not easy but our girl was quite old and used to eating dry for years and years. Hopefully if your boy is only 9 it might not be so ingrained in him. Good luck.