Uusual blister on bottom of foot

My 28-month old son has something odd on the bottom of his foot. It basically exactly like a blister in appearance, translucent to opaque, white, bulbous/puffy, soft, redness at the edges, but instead of being round, it is linear, about 5mm or so wide, three or four inches long. It starts on the inside edge of the bottom of the foot, about mid-heel, crosses the entire bottom of the foot and goes up the outside of the foot about another inch. The line formed is about an inch forward of the base of the heel, the back edge.

There was no event that led to an injury to our knowledge. Inside his shoe, I feel nothing out of place. He doesn't complain of pain but he is obviously walking up on the ball of his foot and not putting the heel down, especially on bare floor but also on carpet. However, he does land on that feel and does not express pain verbally, nor does he wince. It has been three days and no sign of improvement. I no longer recall exaclty when I first noticed, but it might have been first thing in the morning when dressing. Inside the house he is barefoot, outside in socks and shoes. We have a mix of carpet and bare floors. He is not one to jump up and down but he does scamper up and down the hall.

Any ideas?

I have some pix of the foot but need to find a place to host them as I don't see a way to attach them to this message.

thanks all

Uusual blister on bottom of foot - 28 months old