4 year old toe walker

My 4-YO son has been walking on his toes, since he started walking. Not on tip-toes, as everyone else has noted, but on bent under toes. When we tell him "flat feet", he immediately switches and walks normally. But soon enough, he goes back to walking with his toes under. I am concerned about the development of his feet. He walks normally while in shoes (not on tip toes), but the minute his feet are free, he's got his toes bent under.
Otherwise, he is a funny, loving, friendly kid. Making lots of friends in pre-K and no social behavioral problems.
My pediatrician has recommended leaving his shoes on, but he knows how to get them off and does whenever he wants (we have tried, but keeping them on turns into a battle).
Any advice?

[aMommy2one]

My sister and I both done this as a child - Im not sure why, but I can remember it like it was yesterday.  I do remember my mother telling me that we only done it for a short time.  Our feet developed fine with no problems.