I think the other thing going on, making his thighs close --apart from the testicles retreating/disappearing, obviously-- is that his pelvis bone is widening, giving him female hips (body fat redistribution helping that along). That changes the angle of the femur bones a little bit, female vs. male, so a knock-kneed sit would be a bit more natural for him, at that point, especially when he feels the stimulus of his balls going bye-bye.
I'd love to see a rapid-but-gradual MtF change like that today in a Hollywood movie, in that sort of public-embarrassment context, using a combination of some practical effects (as was done here) and some more modern, high-quality cgi (so we'd get some subtle face-morphing, shrinking, narrowing shoulders/ribcage, etc)... with it completing before he has a chance to escape!
