I don't believe in the "clean your plate or you can't leave the table theory." I know it was popular when I was growing up, 50's, 60's, early 70's, but my parents never subscribed to it. I was a very picky eater until I was at least 12. I have a vivid memory of my mom forcing me to eat a banana and drink some horrible chocolate looking goop that was supposed to help my digestive system, which had basically broken down because I was living on bread and butter (Wonder Bread?) and milk. Boy, that really helped .
I had the same issue with my oldest son. He weighed 5.7 at birth and was consistently in the 10th percentile for weight. He pretty much lived on pbj for a few years, in addition to some breakfast foods. It became a real battle between us and was ruining our relationship. So, I essentially gave up. I served fairly healthy meals, but I never forced him to eat anything, including veggies, fruit, etc. He is still a skinny adult, at 118, 5' 6.5" inches, but he eats most foods. And, he's healthy.
If I have leftovers, I take them home. I don't bring Tupperware, but it's a good idea.