>>536538140
Since I am very invested in this thought myself I have thought about it more than a normal person should and came to the conclusion that while holding his legs up by yourself with your arms alone might not be possible because of how fucking heavy they must be, the next best thing would be for him to hook his feet against something like say, the headboard and from there you use your upper body weight to hold him in that position by holding onto his thighs.
Unless of course, he shrinks down enough to a level where you're able to manhandle him without much issue.