Being in South Africa has nothing to do with it. In fact, someday I hope to visit South Africa.. I have seen photos of your farms and ranches.. they are really a sight to behold.
Here in the US a new Saddle can easily push $800 and that is a pretty sum. If a horse colics it can easily cost that.. and a lot more (if surgery is necessary). That is all I was implying and, really, it was meant for you to consider. Maybe vet services where you live are much less and/or saddles are much more.
Now.. back to the horse and training.
Was this horse ridden and trained to ride before you got him? If not, then here we have what appears to be an unknowledgeable owner/rider coupled with an untrained horse. A common saying is "green horse and a green rider = black and blue!"
To be safe... and that is very important.. you really need some riding lessons.. from someone who has been taught to ride (not someone who has "been riding their whole life"). This will help you understand the aids and how to communicate what you want to a horse that is trained. Meanwhile, your horse needs to visit with a trainer who has a proven record of success (not someone who had 'ridden their whole lives and trained a few horses' but never had any formal training (or competition experience) from someone knowledgeable). That would be your best bet for a safe future for you and this horse.
When an untrained horse is first ridden they can react a number of ways. One is to explode. Another is to run away. Some actually stand still and then walk off. Some, like this horse, just do not move at all. Usually good ground work and preparation can mitigate a lot of the xplosions and running away and standing still (but not always) and turn it into standing still and walking off.
The horse that refuses to move the first time he is mounted is usually discombobulated by the feel of weight on their back and how it changes their balance. Such a horse, when they move, can suddenly explode because it is all very hard for the horse to process.
When I trained a horse and got one of these afraid to move horses, I would usually use a leading rein and get the horse to take a step to the right or left in a circle. I had a bosal on most of the horses I started so the pressure was a little different than either a bridle with a bit or a halter. The circle and turning did two things for me.. got the horse to move and also helped to mitigate an explosion or running away!
I still suggest you take the money you are saving for a saddle and spend it on riding lessons (so you can learn how to handle and communicate to the horse what is wanted) and send this horse out to a trainer. When the horse is ready to be ridden, I then suggest you take lessons on your horse.. since each is unique and even 120 days at a trainer does not mean the horse is "trained."
That is what I suggest. Good luck!
There is something about the outside of a horse that is good for the inside of a man. ~Winston Churchill
) Dinosaur Horse Trainer