He just feels comfortable is all. Horses really love to canter it seems.
When you go back to trot, do you do a sitting trot or rising trot? If you do rising (which I recommend to start off with !) you can sort of push more weight down the insides of your legs, exhale and you will feel heavier so he will slow down along with the progressively harder cue to trot with your reins and lack of following seat.
I think you should go back on the lungeline with your hubby lunging you for a little while. Work on that transition.
Another way you can do it is mark a spot in your arena that you always wind down on. It'll get to a point where your horse will begin to antitipcate it, so he'll slow down and trot when you get to that point. Then it's a point of slowing backing that point up farther or shorter so you start winding down before that given spot, or a little bit after. He'll slowly learn to listen to your cues. That's what I did when I taught Sky to canter on the ground. I picked a designated spot and once he began to trot there automatically I made the spot closer, then farther away, etc. It really helped him.
When you canter, you should be consistently asking him to keep a certain pace. If he gets too slow, add leg. Too fast, a few little halt halts. Eventually he'll get to cruise control stage where he manages off of your seat alone but for now you have to sort of be a little more instructive, if that makes sense.
Best of luck :)
"Strength is the ability to use a muscle without tension"