Do keep in mind, bottoming is very much mental thing(65-75%) (35%-25% stimulation of prostate down there).....
If he finds/knows you are busy in trying way too hard to make him cum hands free instead of enjoying it urself...It may most probably not happen...(he would love it though that you are putting extra effort but if you don't enjoy, he knows you don't, so he won't, and hands Free won't happen.
There are actually two different ways I can identify it happening for me, but I always have to be riding, as it allows me to be in control and do what feels good. The first way it can happen for me is if I'm super horny and really into it, I'll feel my dick throbbing as I'm riding, and my downtown clenching with each throb. Eventually I will just start cumming, but I have to be in the right mindset for it.
If your dick is curved you have an edge. Missionary if curves up, Doggy style if curves down, that's all I know.
And remember The more you enjoy the more he enjoys..Is the key. "For a bottom, there is no feeling better then realising your Top is enjoying it". Not even hands free ejaculation. And you will get extra points for it if it happens But do not forget those main points that lies within the key.
Edit : Don't let him cum for some days (longer than usual, you can avoid him and try to make him Jealous so that things get heated up Lol)....And that day he has to be thoroughly prepared so that he can stop being nervous about cleanliness....So tell him at right time , not to early not very late , that you want to fuck him.