to lie on one's belly / to crawl on the belly (said of snakes, crabs, etc.)

to go forward, to advance, to progress / to improve, to progress

voluntarily; willingly

progress

to crawl, to creep

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend

belly

the floor (the bottom surface of a room) / a bed / (theater) a raised area of a stage where a presenter or musician sits