Another thing you could do (if you don't already) is not make leaving a big deal. I give my two bones and then just close the door when I go. they see me leaving as good thing because they get yummies!
Maybe get him a kong or treat ball that he has to spend time getting the stuff out. One of Bella's fav things is milk cartons, I put a cup of biscuits in and put the lid on and let her go to town, she loves it.
Just remember you need to be persistent. Training takes a lot of time and effort.