Is it possible for babies to have sunken soft spot due to stress. my son gets sunken soft spot when trying to sit or stand and its gone when he lying?

Simple gravity: It is probably simple gravity that when baby sits up the soft spot gets sunken and lying down it gets fuller. Sunken soft spot can be sign of dehydration

Pressure monitor: The soft spot reflects the pressure of the fluid underneath.When a baby is upright, it tends to flatten or dip,like a water balloon.Likewise,when they lay down, it fills and may bulge like the same water balloon. If they cry,pressure goes up, if dehydrated, it goes down. If it is ever tense and full, a doc should check baby.

Not to worry: When supine or prone, the pressure in the skull increases slightly which fills out the fontanel. When upright, the pressure is slightly lower so the fontanel appears sunken. Normal at this age.