Horse is humoring you in the pen more than respecting you. The behavior outside of the pen shows it has no respect for you.
The kid analogy I think is backwards though? A child that is taught to mind at home, will mind in public. One that isn't, doesn't.
I agree with tap under chin with boot toe when horse ducks head to eat, and secondly, make sure you NEVER let him eat while leading, or just standing around. It is not fair to tell him "okay" one time, or 20, and then all of a sudden it is not okay for horse to eat.
Also, be alert for horse's signals that it is thinking about ducking head. Head just doesn't fall out of nowhere to grass. One ear forward/one back is indication horse is thinking, and usually, that is not a good thing, as it say horse is reading you and if you are paying attention. A certain distance, mentally from you too is a sign, you should be able to feel this, and the horse will take its eyes off of you, to shut you out too.
With a horse like this, that is basically spoiled more or less, I use a chain shank, and give one pop, and then go back to lead rope. But I expect respect from mine, and they aren't allowed to get by with less.
Make sure horse will yield to your cues in pen, while tacking or leading, and also while feeding. I would imagine horse is showing other signs of disrespect too, that you aren't reading as that.
Horses make me a better person.