When you say he falls over, what does he do? Does he lean over too far and topple or is it like he deliberately falls over? How is his posture when he's sitting? Is he wobbly? Slumped? Does he sit steadily with a straight back and his head held in line with his body? Can he play with toys when he's sitting?
Once he's fallen, does he roll about, try to practise his crawling position etc.? I ask because if he's an active sort of guy, he may simply get tired of sitting - and because he hasn't yet worked out how to move from a sitting position to one in which he is more mobile, his only choice is to just fall over. My LO still does it sometimes when she is cruising and wants to crawl instead - she is trying hard to bend her knees, but sometimes it's easier to just fall lol.