I assume this has already been mentioned, but considering the importance of it, make sure there are no physical things causing this... physical pain, tack, etc. (eg. Vet, chiro).
Don't take your horse to the jump until he is calm and moving at a relaxed, consistent pace. If he is excited, circle him until he quiets down. Circles generally help to calm and rebalance a horse and help to get him in control again. Continue circling until he settles.
Once you are on the straight line for the jump, look up and over the jump- focus on an object that is at eye level in the distance. You don't ever want to look at the jump, because the horse can sense you are doing this and will hesitate as well. He'll most likely refuse the jump- so remember eyes up!
When you are going over the jump, give him a nice release. Make sure that you aren't irritating him by bouncing back in the saddle too early... Things like this can irritate a horse and make jumping not too fun of an experience for him.
Always remember, in order for a proper jump you need to have forward impulsion, rhythm
, and a straight line approach
. Remember that there’s a big difference between forward impulsion and just being speedy. If you bring your horse up to the jump and he's just rushing up to it, chances are your jump is going to be sloppy. Your horse is going to be unprepared, and you might even knock the jump down. The same goes for if your horse is being lazy; if he isn't moving forward with impulsion, he is probably not clear on where he is going and what you expect him to do. This is why you need to get your horse moving with forward impulsion- he needs to move forward at a nice speed, in a steady manner. This way he will be attentive and jump nicely and cleanly. Don't bring your horse to the jump until he is moving forward with forward impulsion.
I wouldn't suggest trotting and then cantering the last seven feet... get him moving in a nice forward canter well before the jump. Circle him until he is moving with impulsion.
Does he just refuse the jump or does he ever run out of it? If he is running out of it, you can place some ground poles on either side of the jump leading up to the jump which will prevent him from running out of it at the last second.
Remember that you are in charge and drive him forward with your leg. Remember that your leg should always be on- asking your horse to move forward- even if you have a speedy horse. This is because if you don't have your calf squeezing your horse forward all the time, if you ask him to slow down, he may just walk or halt. Also make sure that you are driving him forward by squeezing with your calves, not kicking him, which will just irritate him.
In order for your horse to be confident, his rider needs to be confident as well. This is hard, though, when you are worried about whether he is going to go over the jump or not. I'm curious as to why you think he may not 'like crossrails'? It doesn't matter what size the jump is, if you direct him to a jump, it's his job to go over it (as long as it's not physically-restricting, etc., of course).
Before you begin jumping, after your warmup, go over ground poles. Pretend that you are going over the jump and go into your two-point, so that you can focus on your form then. I know it's hard to concentrate on your form when you are concentrating so much on getting your horse over the jump. Keep in mind the important things to help you stay on - always keep your heel down. Don't jump ahead of your horse- if he refuses it, you don't want to go over his head. Keeping your heels down will prevent you from falling off, too. Don't be afraid to grab mane or a martingale strap! All of these things will keep you safe in the saddle. Don't stress out too much over your position right now. As long as you are doing a nice two-point and keeping your heels down, eyes up, etc., you'll be fine.
Start your warmup by trotting and then cantering over groundpoles where you will later on set up the crossrail/small vertical. Go into your two-point over the groundpole. Next, stack up several groundpoles (making a pyramid formation) so that it is not just a ground pole on the ground, it has more to it... trot and then canter him over this obstacle. Next, set up your crossrail in this same spot. When you begin your jumping, pop over the jump in the trot a few times until he gets comfortable. Have him forward so he canters after the jump, and then keeping him in that canter, take him to the jump again.
If he refuses, do NOT let him past the jump, as that is a reward- he gets his way. Back him up, take him to the jump, and give him a smack with the crop. Circle, then go back again. Forward impulsion+straight line approach+a steady rhythm+eyes up+leg on+CONFIDENCE are key.
I would suggest getting a trainer for awhile to help you with this issue, as he is still a very young jumper. When he goes over the jump at the canter, praise him like crazy, let him know how proud you are of him.
Hope this has helped. Keep us updated.
Good luck. :)