I have a 25 year old standard bred who has come a long way in just a few months with a little extra work that's simple enough for anyone to do
At one stage cougar wouldnt canter well either so I worked hard on getting him to trot rather then pace. Its very difficult for them to break into a canter when they are pacing so working on a solid trot is essential and easy ;)
Trot poles trot poles trot poles :) place them 4m apart and do them until he is trotting ALL the time. He may not get the idea at first but it wont take him long to start trotting over the poles and then the more trotting he does the more he will start to trot even when he isnt going over the poles.
Lunge him in a small circle (20m). Once again after a bit of time he should start to trot more consistently as its much easier for them to trot the small circle rather than pace it. Reward all right moves the release. By that I mean as soon as he does what you ask, stop asking him :) that's his reward
As he starts to trot more regularly you can start working on asking him to accept the bit. But that's a whole different thread :)
Work first on getting a strong, consistent trot. You can do it and while it may take some time I don't think he's too old to learn at all. Cougar is coming along so well that my husband is starting him in pony club mounted games soon :)