Impulsion & Tracking Up

I've been working on getting my horse to round up and lift his back at the trot and canter lately. He's been getting it really good at the trot and is now just starting it at the canter

My question is, although I can feel him drop down and reach for the bit, and I can feel his back lifting up underneath me, he still isn't tracking up at the trot. I think it's better than before, but what should I be doing to get him to track up while lifting his back at the same time? If I squeeze and ask him to step out more, his head pops up and he hollows his back most of the time, or braces against the bit.

Are you asking him to step under himself by way of a half halt? Is he straight when you ask? With my horse, if he's not flexed slightly to the inside, (just enough to see his eyelashes) my half halts don't go through. If his head is popping up and he's bracing against the bit when you apply leg pressure, he's running through your aids instead of receiving the information you're trying to give to him.

Repetition repetition and more repetition. You should be riding at the very least 20 transitions per minute that you ride. Forward and back transitions in the gait, and transitions between gaits all count.
You need to get him accepting the aids and really working. Also make sure that you aren't pulling back, your hands should feel like they are resting and allowing forward impulsion to come through. Your half halts shouldn't even cause the hand to pull back.
Also, some horses are never going to track up, they just aren't built for it. If you watch him running free and playing and he's still not tracking up, then it's not your fault you aren't getting it under saddle.

Some horses don't want to do long and low work correctly because it's hard. You need to keep your leg on him when he goes down; if not he'll just flop on the forehand. If his head is popping up when you ask for more impulsion, that tells me that he isn't really raising his back (as the action of raising the back also tips the hip in and brings the leg under), and is just dumping on the forehand.

Keep practicing the long and low; don't expect too much from him, but do not just throw the reins away! Keep him active with your seat and legs; and receive the contact with your hands.

How exactly do you feel when your horse is tracking up? Like, how do you tell?

As Anebel said, do a lot of walk/trot transitions. Each one should produce a slightly better result. As some point, you will feel a change in his body. Your rein contact will feel more elastic and springy rather than you holding up his heavy head. You will feel him pushing himself from behind, and his trot should feel more "floaty". One of the hard things about flat work is that everyone perceives the feelings differently. It's very hard to describe movement. I can only say, when you get it, you'll be able to feel the change.

In the end tracking up is not as important as impulsion. As mentioned some horses will never track up to specific standards. A long backed horse will exhibit excellent impulsion but may not track up whereas a shorter backed horse will appear to be overtracking but upon looking closer it is not using itself (impulsion) to its best ability.

We must also be careful that we don't get rate ( the velocity the horse travels within a specified distance and time) confused with suspension, expression or impulsion which is the energy imparted to each step within the rate the horse uses. Speed does not equal impulsion and just driving the horse forward without care to produce impulsion and increased length of stride is not the answer either.

To the OP...I would be looking at working with the balance of this horse and not quite so much on simply driving forward. Exercises such as turning in multiple directions and doing this slowly will benefit your horse more at this point in time.

I learned one thing while I was working my first horse in dressage and that is ...if you are careful and methodical and concern yourself with getting the horse balanced and relaxed almost everything else will fall into place. The funny thing is that impulsion does not suddenly "disappear' or get lost, it simply waits for the right set of circumstances to show up.