Firstly, Welcome to the Forum!!
You said you were given the western tack that you've been using - was it properly fitted to your horse? Logging miles even in "It Fits Okay" tack can start to adversely affect muscle development. I'll jump on the wagon for having a chiro or an equine massage therapist take a look at him as well.
As far as helping him start to loosen up and relax into consistent and rhythmic gaits, you need to first make sure that you are relaxed. If you aren't relaxed, he can't be either. Try riding to music - I recommend a battery-operated boom box on the rail over ear buds; you still want to hear what's going on around you. The music will give both of you some rhythmic structure to relax to. Work on stretching long and low, to help those muscles from his poll to his croup to stretch and round. There are tons of threads on these kinds of topics all over the Training and the Dressage forums.
I highly recommend taking lessons. Riding lessons, at least in my area, are generally cheaper than "having the horse trained", and IME, most riding instructors allow/encourage pupils to take lessons on their own horses unless there's a reason not to (i.e., need a lungeline lesson and the student's personal horse doesn't know how to lunge, etc.). Even if you can't afford weekly lessons, monthly or every-other-month quality
lessons with "homework" between or emailed questions can keep you and your horse on track and moving forward. I'm not saying it's impossible to do alone, just that you and your horse will come back to form much quicker with some pro eyes on the ground, even if its just a few of times a year.