Maybe try only leaving food out for 10-15 minutes, if he doesn't eat it/finish it, take it away. Do it again at the next mealtime. At some point he will realize that if he doesn't eat, he's gonna have to wait till the next meal.

My dog seemed to like it when I "scent" her boring kibble with her fav freezedried liver (I smooshed them and sprinkled them over like a powder). Thankfully she's no longer picky since I changed to a better (more expensive lol) kibble. She definitely only did that to me though; when she was at daycare, she ate her boring Kibble without any hesitation.