I have a 12 year old quarter horse that I ride both english and western.
He's pretty good at getting his head collected, but I'd like to see him round himself more.
How do I do this? I have a balance training system, but I'd rather not use that.
If his head looks nice but his body isn't rounding as well, he is not collected, in any discipline's sense of the word. Any degree of collection, from western pleasure to GP Dressage, begins in the body and ends in the head. Also, don't worry about "collection" quite yet. If his body isn't rounding, "connection" is of far more importance.
Correct transitions are going to be very helpful to you at this stage. Forget about his head for now. Do transitions between gaits, and within gaits. If you make a lap around the arena without doing some sort of transition, you've gone too far. However, all the transitions in the world aren't going to help if they aren't well executed. You want to prepare him for every change of direction, gait, speed, or balance with a half-halt, and use your aids to clearly communicate and support him through each phase of each transition. Honestly, an on-site trainer or instructor is your best bet to ensure that you're being as clear and effective of a rider as possible -- the devil can really be in the details here, and it's difficult/impossible to say what you might tweak here or there to help more from across the Internet.
Something else to try, to keep transitions from getting dull, is start with some basic lateral work. Leg yields make a useful starting point, although they aren't truly lateral maneuvers, but also look at shoulder fore, shoulder in, haunches in, etc. Those kinds of movements can be great to start really connecting the horse from back to front, and start the process toward true collection.
As far as his head goes, if you're doing everything else correctly, nine times out of ten the head will be right where it needs to be for his level of roundness/connection/collection. No balance training system required.
Sort of a dressage-perspective answer to your problem, and I'm by no means an expert. There are some fantastically gifted and experienced riders and trainers on this board who may be able to describe things better than me... take a look at some of the other collection threads, as well as books/videos by Jane Savoie, Lendon Gray, and Sally Swift. I found that these ladies (among others) have some brilliant discussions on how to get started on the right foot in getting a horse honestly and correctly round, not just have an artificial headset.