This is a bit of an open-ended topic. It's really difficult to say why a pony would tend to hollow rather than go in a rounded outline without seeing the pony and rider in action but here are a couple of thoughts which might give you something to go on.
First, make sure the pony's comfortable. As Skihorse says, are his teeth ok? Does his tack fit properly? If his saddle's tight or restrictive in any way, he'll be reluctant to round his back and lower his head. Bit comfy, not pinching? Browband wide enough?
Looking at the schooling angle there needs to be a fair bit in place before a horse can go in a rounded outline. It needs to be going sufficiently forward - enough impulsion/energy - in a good rhythm, and also straight (by which I mean not crooked, escaping out of one shoulder or another). The rider needs to be sitting more or less correctly (none of us are perfect unless we're C Hester or someone) and riding the pony forward into soft, receiving hands. Yes there must be sufficient contact for the rider to be able to influence the mouth through the bit, but that contact needs to be elastic and consistent. That's what my DCs struggle with at the moment - just being able to keep their hands still enough. Centred Riding's good, and there's also a Sylvia Loch book called "A Kinder Way to Ride your Pony" which is designed for children.
OK...if we're happy about these things so far (and no-one's nodded off to sleep yet) we can perhaps think about some specific exercises to get the pony to lower his head and lift his back into a rounder frame. Will the pony soften his jaw and round at the poll in a halt? If so it seems he understands the rein aid to soften and relax his jaw which is what you need. Then, a basic 20-metre circle is a good place to start. Check your shape, the quality of the pace (energy, rhythm etc), make sure the pony's not leaning out with one shoulder, then gently ask for a softening of the jaw on the inside rein. Once he gives it, the rider can yield with the outside hand, but shouldn't drop the contact on the inside - just hold it soft and steady. The idea is that it is comfortable for the pony so he wants to stay in that outline.
Are they happy going over poles? Walking or trotting poles, especially on a circle, can encourage a lowered head and rounder frame.
How's their canter transitions? Repeated trot/canter transitions, not spending too long in each pace, are excellent at gently stretching the long back muscles which, if tense, won't allow a rounder frame.
There are several other things such as some lateral exercises which are great for softening/lifting the back and dealing with hollowness but I'm not sure if they would be too much for the pony/rider combo at the moment (not being rude - my kids can't go sideways yet)...but let me know if you want more ideas.
Hope that helps.