I agree with Miah, keep your combinations and lines the same length as they are at the show. Start with some ground poles or cavaletti 5 strides apart and practise collecting him down to 6 strides, and alter with the normal 5 strides to keep him confident. Next put up some small jumps instead of the cavaletti, and try the same thing. Making the strides bigger to fit your horse will confuse him and will very likely cause him to become insecure or knocking fences down at the shows.
Gradually work on collection. Start with getting three strides of collected canter then build your way up. There is no easy way unfortunately. I love just setting out canter poles 5 strides and adjusting my horse between 4 to even 8 strides in between. But try not to train in different lengths then shows, unless you're teaching the horse to adjust in between jumps. If that means you have to jump smaller then you normally would because of the stride change do it! It will be a much quicker process then getting to a show and not being able to make lines and combinations