OP,
I was actually in a horse/rider situation similar to yours once. My first horse (as a green amateur) was a coming 7yo with a little less than 60 days under saddle so I have been down this road before and know that it can be rewarding, but challenging! There are some things that stand out to me in your post, and I’ll do my best to comment on them as I think it might be productive for you, but I’m going to preemptively apologize for the wall of text. 
First thing’s first: I know you’ve had this horse for about a year so some of this may not be entirely relevant, but it is worth keeping in mind. We, as people, do not go from “sitting on a couch” to “running a marathon” in one day. We do not have the muscle or fitness for it. Running is hard. Many of us need to be taught proper running form (and even “fit runners”, when challenged to “run in proper form”, may find themselves fatigued more quickly than normal). This is to say, when we try to get horses to “go correctly” often we lose sight of the fact that doing so is exhausting and no horse is going to be able to go from “not doing it” to “being correct” a majority of the time. Progress will be tiny, tiny steps that are building blocks for the entire affair. You do not start day one on “he is using his hind end correctly”, either.
Which is to say that “using the hind end correctly” is a good step in the process of building muscle over the topline, but 1. it is not a starting point in and of itself and 2. it also will not be the only thing you have to address. Think of all of the moving parts that are involved in “hind end going correctly”. There is joint articulation, there is lift, there is reach, there is energy, there is rhythm, and there is relaxation. It’s absurd to expect anyone to just go “well now, I suppose it’s time to have a go at using the hind end correctly!” (Also, refer to paragraph above - it is hard for horses and it is natural of course, they would much rather not exert the energy or effort.)
This also brings to point that you can have as optimally working a hind end as can be found, but unless the horse is using themselves correctly, it will all be for peanuts because horses that generate activity and power from the hind end can still drop through the back, or evade with the shoulder or neck. Another post above mentioned it, but both of these (neck and back) are entirely suspect areas for stunting development because many riders have a tendency to want to ride from the front to the back (rather than back to front). Stifling a horse with our hands is an unfortunately natural reaction for a lot of riders.
There are a number of really nice exercises out there - some (transitions) will help sharpen up the hind end. Cavaletti (Ingrid Klimke has a great book on exercises, I love it to death) can help build power. However, unless your horse is working correctly from the get-go, these things at best are going to have minimal effect and at worse, just be a hard challenge for a horse that won’t get anything from it.
You don’t mention it in the post, but how is your horse’s ability to use his back/stretch into the contact? One of my favorite things to do to build muscle, honestly, is just walking. An active, swinging walk that stretches down and OUT into the bridle. Doing this out in a field or trails is a great, low-pressure way to help develop a horse. (Hills are fabulous, if you have them.) And then taking this idea into the arena, doing work in a “stretchy” frame - some people may argue that the task is to get the horse up off the forehand, but until they are relaxed, have good cadence, balance, and connection, you may not really be at a point to look at more advanced self carriage. (Keep in mind: stretching can also fall into the category of “This is HARD WORK,” you need to build up to your expectations. For a horse to have a really extreme stretch out and down - the OUT is key, mind you - they need to develop that muscle. And of course, a part of the stretch will involve the horse engaging the core and lifting the back, driving with the hind end.) Stretching is truly an underrated option for helping develop a horse…
How familiar are you with the dressage training pyramid? If it’s known to you, I would strongly encourage you to go back and review it - if it’s new, then take some time to do some in depth research. While it’s not necessarily a concrete “This before that”, I think it does a great job illustrating how the more challenging/advanced concepts build on each other. Especially with green horses, making sure the foundations are good before pushing for more (like self carriage, a more uphill way of going, etc), is key.
…And then of course I also echo everyone else. A “teacher” who criticises but then offers no specific area of challenge and fails to offer a route to improvement, is generously, a terrible teacher. More frankly, they shouldn’t be teaching. If it was a bad day, I would consider asking them to specify - what that they see is terrible? Ask for details to help you train your eye and develop your plans. And then ask them to help you work through how to bring the horse along to improve those areas of weakness. If they won’t engage in these conversations, it’s time to find a new professional to work with.