And so we say the thing is impossible.

But is it?
Only under the relative conditions of his mass and the earth's.

If the world he happens to inhabit were not its present size, but the size of one of the tinier asteroids, no such disastrous results would follow a chance misstep.

He could there walk off precipices when too closely pursued by bears--if I remember rightly the usual childish cause of the same--with perfect impunity.

The bear could do likewise, unfortunately.