But with no rope around belly~he wont go down if I just hold hoof up myself
Right now it seems that the rope is the cue. Since he's already lying down on cue, you could just slowly fade out the rope.
I don't know if you are using clicker training, but I guess you will use some reward once he has laid down. So I would try to simulate the rope with your hand and at the first sign of going down (smallest weight shifts, etc.) I would reward him. Then slowly ask more until he lies down without rope. You can also try to alternate using the rope and then no rope to get him on the right track.
If you want to build up the behaviour from scratch, there are several ways you could try. Some people go via the one legged bow, kneeling and then lying down. It is a bit of a detour, but I guess it's intuitive for some horses.
With my horse, I used a combination of capturing and mimicry. I rewarded every idea of lying down, when he wanted to do so and I was around. Then he already knew that I liked him to lie down and so I could install a mimicry cue. I tried to imitate him when lying down (walking in circles with head down, shuffling the sand with my feet, bending the legs, etc.) and lately I added a cue with my hand as well to fade out the other cues eventually.
With Mucki it was quite easy though, because he loves to lie down in the arena
In this video you can see what I just described: http://www.youtube.com/watch?feature=player_detailpage&v=-ki6Jrf9WCI#t=110s