Hi,

It is normal for hatchlings shells to be a little soft
on the underside as well as back by the tail. If you
indeed have the uvb souce and are feeding it Reptomin
there is little chance that he has soft shell disease.
These areas of the shell will harden up as the little
guy gets older.

I would not worry at this point but just keep an eye
of him. Attached is a picture of my 8 year old that had
the same thing when she was a hatchling. Never been sick
in her life and she now 3 lbs!

Hope this helps.
Linda