I only ride this horse, he is not mine... :)
How can I make him use his back more?
And what do you mean by "be on the bit"?
That's called hollow.. especially when paired with an upright neck rather than being level or stretched down neck. It's very tough for the rider to sit quietly on a hollow horse.. and the horse isn't very comfortable either.
You want the spine to be curved upward like a small mountain and you cannot force this.. the horse's neck will begin to stretch forward and engaged so that the top of a horse's neck (where there are muscles) are bulging and working instead of the underside of the neck.
It takes a lot of preparing and practice but it'll become easier for you to know how to ask and for the horse to happily travel in this manner.
Here's a pic of an engaged horse/rounder horse. PLEASE ignore the rider... ahaha this was before lessons:
Okay, so this rider sucks (it's me lol) she's leaning forward, she's throwing her hands away. But look at the horse.
The first step to get the horse to move properly is the rider has to ask the horse to move FORWARD. Leg is on and notice the pink markings. The pink dashes show how far UNDER the inside hind leg comes. The hind of a horse is the motor; it holds all of the power of the movement. The farther under the leg comes, the more POWER and ENERGY you are asking for (do not confuse this with speed.)
Once the horse is forward, the rider then needs to stop the new energy from escaping. The rider accomplishes this by closing their fingers and then asking the horse to soften. When the rider closes their fingers the energy is caught but then the energy cannot travel anywhere so the rider must encourage the horse to stretch down by following the horse's movement with their elbows and playing with the bit in their mouths. DO NOT PULL THEM INTO AN ARCHED NECK!!!! And DO NOT LEAN ON THE REINS!!! You will have to develop your own "feel" for it. Instructors are helpful also. When you accomplish this, the horse will stretch its neck down on its own. The top of the neck will bulge and then arch as shown by the blue arrow. These top neck muscles will pulse as those muscles are being worked.
Once the horse is forward, the energy has been caught, and the horse begins to work those neck muscles, the rider must follow the horse's movement with their seat, not bracing just being nice and loose. When the rider has asked the horse, and the horse continues to do ALL of these things correctly, the spine will lift/curve as shown by the green arrow.
This is a round horse. An engaged horse. Though not quite on the bit.. on the bit is when a horse is reaching for the bit and the rider isn't throwing away the contact like I am above... but even so.
Both of these adjectives describe a horse that is a lot easier to ride, and is overall more comfortable (though round is a lot of work so it takes some time.)
And this goes to show that even a crap rider like myself can prepare my horse to be round
A good thing to note is YOU MUST CONTINUE TO RIDE IN THIS MANNER! Or your horse will slowly go back to hollow. It takes practice.. so don't be discouraged. Your #5 was close to an engaged horse.
There are some tips on this website that will help you:
::: Sustainable Dressage - Rollkur - How And Why Not? - The Shoulder-In Volte for Stretching Over the Back :::
Just know that when you ride, you should not lean or brace against the horse. Move with the horse, push the horse forward. Instead of pulling back to stop or turn, use your ab muscles to keep your reins and simply stop all motion in your seat.
I hope that helped.