If pressure on the leg doesn't work then get a crop and tap them not too hard 3 times right behind where your foot is placed. Then the next time they should keep going when you put pressure on. I also ride a lazy horse and she has been getting a lot better lately.
My mare does the same thing. I use a crop but don't actually use it. I just make sure it's moving slightly against her body. I don't hit her with the crop, just keep it barely swinging against her. But also make sure you're not holding him back from continuing to canter
Make sure you are not opposing his motion and pay good attention to your following seat. Initiate the downward transition to trot after five canter strides then upward back to canter again . Increase the period after progress and make sure not to lose the impulsion while doing transitions