Does he know how to yield his front and hind end? Side pass? Spin? Leg yield? Cow literally lives up to his name and used to be quite dead to my aids, I started working on yielding his hind end with using the fence then eventually turned into side passing. It really helped him get off my leg more. I noticed you said you tie him up, I tie cow too when he used to have hissy fits (also cold backed, went on advice of a clinician) which I also noticed softened him up, except for the walk it ruined it for us haha.
Do you ride with shorter reins? I just started loping my horse this weekend, and well not going to lie little nervous knowing his past (one buck and your off no matter what, plus my coach had a broken ankle from him but separate occasion) but by sticking your hand down on his neck and just letting him go his speed till he relaxes is probably best thing we've done yet.. His head was naturally long and low and just WOW, what a jog I got from him didn't need to figit at all afterwards. If lope is what you need to work on my advice is just do it! :) have someone on the ground if your worried and helps with the support when They let you know it's ok he looks awesome keep it going.