Can't canter him while riding.

Well my pony aussie will not canter when being riden, he can canter fine in the paddock and gallop but just not while being riden. When asked to canter he does a really fast side stepping trot.
He is very fat but we know he can gallop so that can't be his problem?
I have not tried cantering my mum and cousin have but I am little then them should I try to? Although I don't think im that confident yet.

That could be your problem right there. The horse knows you are not ready. You may be pretending to want to go fast but deep down you are holding back...they know that stuff. I have the same issues with my big old lazy fat butt. She just hasn't been asked to do very much in her life, and cantering is too much work for her. I found that carry a crop and giving her a tap has started to motivate her a bit...plus lots of work on getting her running out of the saddle, showing her running is fun...and getting her body in shape to exert itself past the comfort zone. She is soooooo lazy sometimes when she walks it looks like she is in slow motion...she even swishes her tail in slooooowwww motion. When you are ready...let that pony know you mean business...start making it move faster...don't let it be so lazy. Ride it like ya stole it!

I have not tried to canter him though only they have?

Are the more experienced riders getting him to canter? If so, then it's you, not him.

You're probably not balanced enough and don't have the right cues to ask him to canter.

An unbalanced rider is hard on a horse, especially at the faster gaits. You can even throw him off balance if you're not experienced at the canter.

Sounds like you need some lessons.

I second lessons. Lessons are essential in teaching people the correct cues needed to make the horse understand what you want.

Confidence is key in horses. They want to be shown the right way to do it.

Besides that, perhaps ask for a canter out of the arena, such as on a trail or something, maybe slightly up hill. Cantering on a circle, or on uneven ground, can make it much harder. If your mum and cousin are more experienced, then perhaps they can teach you?

Ive had lessons, and my cousin and mum are pretty experienced, I have cantered a horse on my last lesson up a hill and I really enjoyed it, on the weekend if nice weather ill get my mum to canter her horse in front of me and ill try canter behind because he is a bit of a follower. In our paddock one side is a bit of a slop so we will do it going up there.

My son's pony Misty will not canter under saddle. I believe she was initially broke under harness and was taught NOT to canter while working. She will do a furiously fast high stepping trot, but after a lot of work the ONLY way she will ever canter even now, is on the trails, when everyone else is cantering, she will do so to keep up. Still working on her. Lots of lungeline work at this point.

Sadly aussie can't lunge and when we have like a crop or flick a rope he spokes because we belive he was bashed when he was little with old owners.

Why do people always assume a horse has been abused when it doesn't want to do something? This completely and totally drives me up a wall!

If you don't have solid, verifiable proof that the animal was abused, you should never, ever state something like that!

I'm betting the horse has never been abused, and just acts like an idjit because he either doesn't know how to lunge or just doesn't want to.

Even if the animal was abused, there's no excuse for him acting like a heathen and your letting him get away with it. I get sick of people making excuses for their illmannered pukes.

What the animal needs is proper training and discipline, not a pat on the head and a treat when he acts like a freaking moron about something.

God I run in to this CONSTANTLY with dog training. People have fearful or reactive dogs and don't like it when I tell them it's their fault. I usually make an abrupt example by waving my arms and shouting, the dog freaks, and the people bend down and hug and kiss the dog, saying "oh, poor baby, it's alright..."

Then I just look at them and tell them, they just rewarded their dog for getting scared. They are TRAINING them to be fearful.

How the basics of this completely escape people is beyond me. I've never understood how people don't just "get it". It took me years to figure out this understanding doesn't come naturally to most people.