Ideas? Opinions?

While nearly every time I ride I concentrate on keeping Milo round and lifting his back, it seems he either a) doesnt quite understand what Im asking for when Im cueing for a "rounded back" or b) doesnt care and hollows anyway or c) Im asking for it wrong or not going about it the right way.

Hes got it down at any walk, trot, extended trot, jog, or stop. And what I mean by that is a supple, soft, round face and back. But when I bump it up to a lope (even just a few strides) he hollows out in the take-off and only relaxes and rounds briefly, and usually so little I can barely get a praise or reward in for him before he hollows out or speeds up again.

Is this something I need to just do a LOT of transitions for, or do extended periods of loping? A rider at the barn suggested ground poles or cavaletties to encourage him to pick up his back. If thats a good idea, what would be a good "set-up" for the ground poles? Im not too savvy on working with ground poles (crazy since I used to jump) and not sure about the right way to approach them with a green-to-pole work horse. (Dont get me wrong, I have worked him over poles before, on the ground (longe) or under saddle, but usually just one pole).

Any suggestions? English and western opinions are valuable. :)