Beg.

I'm not sure how it happened, but the big kids have discovered - and, shall we say, exploited? - Luke's soft spot for dogs.  They've figured out that if he has something they want and he's not interested in sharing, they can change his mind simply by acting like dogs.  The below picture is NOT posed.

It usually goes something like this:
Luke is eating a treat somebody gave him.

Big Kid: Luke, can I have some?
Luke: No.

(Big Kid gets down on knees, shows paws, pants, woofs, walks around on all fours, etc.)

Luke: Here you go, doggy!  (Immediately shares the treat.)

The best is when all three big kids start acting like dogs in order to get a bite of a cookie or a couple of Skittles or something.  I'm not sure if they've trained him, or if he's trained them?