The hey to jumping these fences straight is to use your legs as a chute’ and make sure the horse is straight. He can be straight on a curve, for instance, when riding the dog-leg, but his body must be straight in that his hind feet have to follow in the same track as his forelegs. This requires you to work your horse from the leg into the hand and should be established in your flat work before you start jumping. It will also help you when jumping - there’s nothing worse than having a horse leaning on your hands and pulling like a freight train around a course!
Are you putting your horse in balance around the corners and getting straight to the fences? Riding a bad corner will result in a bad approach to the jump and possibly a bad jump, too. With young horses, it’s imperative to get straight with a nice approach and have balanced take-offs. If he’s drifting across the fence, or gets in too deep or long, he will have to try harder to jump and this could knock his confidence, resulting in refusals, especially if you have a particularly careful horse.
Another habit that will benefit you when you get to the show is to teach your horse to land on the correct leg. Look where you’re going and open your inside rein over the fence - this will encourage your horse to land on the correct leg and you’ll be in better balance on landing, ready to take on the next jump. This is especially helpful when jumping the dog-leg. While your horse can jump off the ‘wrong’ canter lead, he’ll be in a much better balance on the right leg. That’s a big part of planning ahead, knowing where you need to ride forward, change leg or turn to be straight on to the fences.