Your horse must be comfortable with you being around it while it's on the ground. This takes a lot of trust (more than people would think) horses don't lay down if predators are nearby and we are quite literally a predator, start by being around your horse when he rolls, don't just let him go, walk him around and when he roped to roll move towards his had so he doesn't kick you accidently and stay close by so he can learn to feel comfortable on the ground with you nearby.
Teaching a Bow
To teach a bow, find your horses absolute favourite treats, the ones he will do ANYTHING for. Start by putting the treat between his leg, say "Bow" and say it till he realizes the treats there. You want him to simply look between his legs and take the cookie out of your hand. Next bring it a bit farther back and repeat this several times. When the horse gets that you're feeding him between his legs, you introduce the next step. Ask your horse for his foot, not hold it up, put the treat with your other hand through his legs and say "Bow" he should look for the treat between his legs as you've taught him. Just let him take the treat a few times jump holding his leg and not asking him to go down.
When he's doing it a lot and is comfortable you want to put the treat farther under his belly, ask "bow" and this time if you've moved it back enough he will begin to lower himself to get the cookie. Do this repetitively slowly increasing how far down he must go for his cookie. When you've got him so he puts him leg down onto the ground for the first time reward him and end that lesson, let him know that's exactly what you wanted and because he did it he doesn't have to do anymore work!
Do this every time you go out to see him and soon he'll learn. As he learns what "Bow" means more and more you'll eventually get to the stage where he will do the actions without you waving a cookie between his legs, or you ca introduce a specific tap on his leg or shoulder that signals to him what he should do. Just replace the word "bow" with the tap or a click or whatever you want. It all takes time! You can't get a bow in a day ;) As for laying down, I've never done it, I don't feel like doing it to my horse. But there are many videos on youtube about how to do it, but your horse needs to feel comfortable, don't force him to go down or he might become dangerous.