if you havnt ridden this horse in 5 years, I think it could be lack of fitness? I've re-trained a pony, last year, that hadnt been ridden for over 2 years, I started right from scratch with the riding side
The go and stop button (in your case its the go)
Stering (so he nos to stay on a circle of a straight line, when your legs and reins ask him)
I will then work on his transitions (walk to halt, halt to walk, walk to trot, trot to canter, back to trot, back to walk, back to halt) yeah you get what I mean haha
Also lunging will be great to his fitness, as well as letting him relax on a trail ride, and that will improve his confidence, and give him a break to just be a horse! Lol all this training may be making him feel like a robot. My horse gets VERY sour if I do to much dressage, so once a week, I let him go for a nice canter on the beach, lets him relax and have fun.
Also I do agree with Jazzy, the balence could be an issue to. As she said do some spirals at the walk to trot, and serpentines (loops) and figure 8's.
Shetlands are also naturally LAZY ponies. So id expect them to not canter long distances. Make him canter 5 strides, then back to a trot, and give him a reward. He will then learn to love cantering, and his canter wont be so FAST, and he could slow to a nice pace with lots of rewards, and different things to do.