I am not able to downsample the default number of vertices i.e 15002 as the volume source grid is not showing up while selecting the head model.
It seems here that the is some confusion between surface head models (using the vertices as the source space - downsampling the cortex surface creates forward models with less dipoles) and volume head models (grid defined at the time of the head model computation).
The head models, volume grids or source results can never be downsampled after computation.
https://neuroimage.usc.edu/brainstorm/Tutorials/HeadModel#Source_models
https://neuroimage.usc.edu/brainstorm/Tutorials/TutVolSource