Hi there,
As Elora said if he was underneath his basking light this is completely normal.
They puff their bodies out to absorb the heat and relax.
We tend to refer to it as "the beardie pancake"
As to the belly colour being all white, white is actually the colour belly you want your beardie to have all the time.
A creamy white belly means a happy dragon ^_^
When your beardies belly has darker lines on his belly it is an indicator that your dragon is grumpy. Longterm lines can indicate health problems.
Do let us know about your set up but it sounds like you're worried because you have a healthy dragon! hehe
Holly