How to get a horse to stay in a trot or canter?

use a crop. You can give a lighter tap with a crop than a kick and typically its more affective. when he starts to slow down like hes going to fall out of it, give him leg pressure but no kick, along with a click of your tongue, if he doesnt respond follow up with the crop. if he doesnt transition downward and continues trotting or cantering, only make him do it a few more strides then lower his transition as a reward. do a lot of transitions with him so he doesnt think youre just going to make him trot or canter in endless circles.