typed up a response and the stupid forum EATS IT! Ellie is about the same size as Luke I think, just shy of 20lbs? maybe 12" tall?...she loves tennis balls, and will try like anything to catch them but they are JUST a smidge too girthy and almost always pop off a tooth and away. When she has one on the ground she can easily pick it up because she has the time to adjust her jaw. On the fly she just cannot do it. You might have a similar issue with Luke? He might not even like the feeling of something bouncing off his teeth/mouth if he ever has something tossed at him.
I might start with food on a long blunted skewer...tap his muzzle and mouth with it and I bet he'd open up. When he does say CATCH IT! Bring it in from above, below sideways etc and say that phrase the second he opens his moosh to get the food off the stick. Use something he really likes and doesn't haveto lick, like marshmallows or hot dog pieces. He will probably start to grab at the food...then perhaps the transition to CATCH IT without the stick or perhaps from a more unpredictable moving string type set up??
Other dogs learn by watching or from excitement...have another dog catch food in front of him and don't let him get in on it. Make sure the other dog isn't snarky tho around food of course. He may "get it" then or be so stirred up that when you let him go he just tries out of excitement.
Once he is sorta trying for food or even doing food...try rubbing a small ball (large enough not to choke him but small and light...maybe a ping pong ball? and toss it saying CATCH IT!...
Perhaps then you can move up to a tennis ball and have that command and he'd know what to do?