Get him back!
To get him back something needs to change. If he's gone, it's because you two reached the limits of what you could share together.
To take it one step further and keep evolving together, you need to bring in something new, something you did not do before.
Forget about force, ultimatums, etc. Modern human rights claim the power of self determination. I agree!
Everyone is free. Now imagine that within that freedom, he realizes that what you have to offer is the best thing that could happen to him. Shift your perspective and think for second: what is the best thing that could happen to him? What do you feel is his next step in his personal evolution.
Don't use control! Set him free instead! If he feels free, he'll have no problem spending time with you.
You see, we can't cheat with someone's destiny line. We all have a powerful spirit overseeing our lives and bringing us where we have to be. Sometimes these lines open up! Sometimes they don't! True
The best thing is to "partner" with his spirit and give that man what he most needs! Are you in his future or in his past. Are you part of something he had to experience in the past or do you have something significant to offer him in the future.
There is a great logic in life! It's called synchronicity! It happens when two people truly meet: common needs, common desire, same life vision. WIN - WIN always.
If you feel you are not in his future right now, what could you do to be in it. Take a deep breath and start taking steps to be there. It's an incredible opportunity for both of you!
Don't think you are doing that for him! Do it for yourself!