Then you need to start with a proper measuring not only for the base ring with the diameter and length of the tube. Not everybody has their different wants on how they want it to happen I personally believe that no matter what position he he is in standing or sitting leaning forward with his elbows on his knees in his most least excited condition he should still reach the end of the tube if this is going to be long-term wear just for personal hygiene shall we say so that once inserted he stays pointed out the exit and doesn't turn into a lawn sprinkler trying to go to the bathroom. Like I said something smoother like what you were showing in the video is a good starter for the tube as provides less discomfort should he get excited and have to deal with a redirected blood flow. Definitely check all edges whether you are going with plastic resin composite or metal for any unsmoothness or burrs and address that with sandpaper for comfort and start in the very beginning with only a few hours at a time and maybe at home before going out into public just to get used to the feel of this new situation
