R.S.
It's about the age when kids start lying, trying to manipulate the world to their liking. Just never let him get away with it, so he'll grow up thinking you can always tell. My 16 year old daughter lied alot when she was little, and she'd always shift her eyes and look kinda guilty, and I told her that i always knew when she was lying. Now she doesn't even try lying, but she will leave out the parts she doesn't want me to know (and I find out anyway!)
I'm more concerned about bribing him to eat by offering chips. You can't let a kid eat all junk, but making him eat more than he wants to get the treat food isn't a good habit, either. Try giving him a smaller sandwich and three or four chips, and then if he's still hungry another piece of sandwhich and several chips. That way, the chips aren't a treat, they're just part of his meal, and he learns that we eat chips in moderation and enjoy them and we eat healthy food and enjoy it, and that we quit eating when we are full. Most kids would eat all chips if you'd let them, but by giving him a few to start off, he's more in control of what he eats and he learns better habits.