He is pretty young so a lot of horses need to learn balance at this stage. Of course you'd want to make sure that this isn't a lameness or pain issue first. But assuming it's not- So when I first started working with young horses a few years back, I read these books by Cherry Hill, she has some great suggestions. Ground exercise wise she has one called 101 arena exercises plus a whole lot more that are under saddle.
Anyway, some of the basics I got out of it were (and most of these are a practice practice practice kind of thing)
1. Horses have to work harder to stay balanced in a circle, so lungeing is good for this. If he's already good on the line, then long lines or the Pessoa system can sometimes help him use his hind end better. Your trainer might have these things. Under saddle you can do lots of 20 meter circles to help him learn, just be careful and don't let him drop his inside shoulder 'cause that's kind of cheating, maybe your trainer can help.
2. Serpentines are good for balancing through the turn and for bending, so 3 or 4 loop serpentines across an arena. Again the shoulder thing.
3. Uphill is great to get a horse to use its hind end, so if there are any in the area you can trail ride and still be helping him.
4. Transitions transitions transitions. Transitions in the corner too. Walk-Trot-Walk. You can ask for a walk-canter transition in the corner and then bring him back down to a trot or walk on the straight, and do it again. Or keep him on a circle. The options are endless.
Just wanted to point out also that you don't want to do the same thing over and over, that's why I made multiple suggestions. If a horse gets bored or does the same thing again and again, he will start anticipating and that is not the point, the point is to be responsive. So before you ride, make a goal of what you want to do that day and think ahead. If you have a plan it's easier, and then switch it up between days. You don't have to work on everything all at once.
Anyway, good luck! With the young ones practice practice practice...and patience!