Sinking swimmer

I am helping a relatively new swimmer improve their technique. Stan, is a strongly built swimmer with NO natural flotation. When he swims his whole body is below the surface. He holds his breath for many strokes, and has to work on his kick a bit to raise his legs and improve his body position a bit more, but the main issue seems to be his general sinking. I would expect the breathing, kicking and head position would affect him being more horizontal but the sinking is hard to change. Any suggestions would be appreciated.