To get the hole in the face and in the helmet, I've created a polygon object and used it in a boolean operation. After getting the hole, I corrected the face model to get it back quads again.
After doing this, I started the modelling of the skull, using the poly-by-poly method and some references I gathers, I ended up with this result. So I can now apply the shell modifier and use this as a base mesh in Zbrush.
After modelling the helmet and a shirt for my soldier guy, I ended up with this final model to take into Zbrush.
Ok, so I have exported the head and the skull model into Zbrush in order to add some more detail to them. Starting with the head, I've put some scares and skin pores, and also some adjustments in the facial expression