for the sit, just put him back in the starting position, or get a cheep door mat and teach him he only gets the treat if he stays on that.
do you get him to sit stay before ge gets to his dinner?
you just need to work on his respect around food. hold a treat in a closed fist, he doesnt get it till he leaves your hand alone, never let him get the full treat in one go only let him have little bits. the probem can be that once they get the treat they dont think they have to behave anymore. where as if they only get a little bt they know their is still more so thing stil have to listen to you.