I know it is hard to be a working mom. I was one for years. What you feed your son will make a healthy and productive postive or negative difference as he grows. It is good to hear that you are trying to learn what to feed him. His future is your responsibility.
If nuts are not an allergic concern for your son, almond or organic peanut butter on whole grain bread is your best bet. How you cut this sandwich is critical. Cut it in half, vertically. Then cut the halves in thirds vertically. What you end up with are "finger" sandwiches - easy to eat and full of nourishment. Fruit, such as red grapes, that can be eaten as finger food is great. How about a little salad combination? Baby carrots! Left over chicken cut into bite-sized pieces, peas and green beans that are not mushy are yummy. I would not be so concerned about the food being hot as it rarely is by the time you have cooked it and then sit down to eat it at home. This is a challenge at any meal. If you want some foods to be warm, use the thermos type lunch packets.
A combination of a different naturally-sourced, God-made, colors of foods is very inviting for little ones - actually for all of us. White, beige and brown are boring - add red and yellow (thinly sliced bell peppers), blue, and green!
Our website, www.familyfoodexperts.com, has lots of recommendations. Our book, Are Your Kids Running on Empty? and the CD cookbook, Mom, I'm Hungry...What's for Dinner? are full of fast and easy solutions. They include the Better Choices Grocery List. I am mentioning these because your question deserves a comprehensive answer.
Let me know how this works for you!
E. (____@____.com)