The information/suggestions are great. Sounds like we will need to expand our "toys." Any thoughts on the mental aspects of increasing his submissiveness?
- get him to call you Sir or Master maybe other things. Occasionally mine call me 'my Lord' ...
- most people only do that during a scene, but some play with doing it at other times, or even with some other people ...
- the collar is a key symbol - it can help a sub feel submissive
- handcuffs ditto
- kneeling is a key thing
- also sometimes make him prostrate himself before you
- a "fuck position" with him on his knees, head down and ass up
- accepting pain from you naturally increases submission. Clarify different types - e.g. "Because you need to be punished / corrected for X", "Because it's time, in your slave development plan", "because I feel like it, boy" ...
- think about a short written contract clarifying the rules of engagement
- get him to bow his head to you, either when standing or kneeling
- clarify how he should respond to orders ; for me that's basically
(i) acknowledge the order / show you have heard it
(ii) begin to follow it - do as you've been told
(iii) do it well, like you are enjoying it, even if it's difficult ...
BUT I also have ways to ensure they can clarify or withdraw the terms of their consent at any point - for me, I make sure I can't accidentally make them do something they really don't want to [ may sound paradoxical, but in fact I'm a kind, law abiding adult ] ...
- another thing might be eg visiting a fetish shop / adult bookstore etc together. Maybe he might be in some gear or visibly collared, but that's not critical. Choosing items together. Maybe he kneels, and you hold a big hollow strap on dildo at your crotch, imagining how it will look and feel going into his mouth ...
... the feelings of submission will grow. And so will the natural pace of your dominance ...
...
Hope that is a start point for you!
Think through your key points and adapt from there.