How do I go about teaching my reiner to lower his head in between maneuvers?  I've seen riders cue for this--bumping with the legs with rhythmn--but I'm not sure how to teach it.  He's also used to me applying slight pressure on the reins and bumping with my legs for a back up, and then, when moving forward, applying rein pressure and bumping with my legs for collection, so I'm sure he'll probably be confused about what I want him to do when I start bumping with my legs.

 I'm not sure (because I ride dressage, and have only been on a reiner once, lol), but they may be bumping to ask the horse to lift the back, which will make the head go down.

 A hollow horse won't be able to lower it's head, so you need to get the back up and round. I wouldn't be surprised if there was a spot to give a little rub (w/ a heel or spur) to say "lift it up." We do a similar exercise off the horse w/ hoof picks (insteadof spurs) to get them to practice using those muscles.

So, for now, if you have any cue that means "be round - lift you back" it will probably work. Just be sure your seat isn't saying "go" (forward or back).

Edit because "hell" and "heel" are not the same

Thanks for the advice.  I've been experimenting with some things, and I think I found something that will work: I bump him gently with my legs, sitting tall so he doesn't think "reverse", and if he doesn't drop his had, I ask him to vertically collect his face, which causes him to automatically drop his head.  Eventually, he should learn to drop his head when I bump with my legs.  He distinguishes my asking him to drop his head with moving forward because I squeeze when I want forward motion, instead of bumping.

That sounds correct to me.  My reiners all lower their head with leg pressure, but if they do not listen to the leg we apply rein pressure and seat at the same time, once the head starts to go down release the pressure.  They may raise their head at first to the rein pressure but do not release until the head comes down.  Doesn't take long for them to figure it out.  But always apply seat and rein at the same time, pushing their butt through to their nose.  That is how we do it my barn!