There are a million possibilities. Considering the amount of information you gave me, it could be anything. But if you took time to answer my questions, I would take time to answer and quite possibly help you.
I agree with Kirsty, you need to give more information for others to help you,
For instance if the horse has come of the track it may be detoxing,
If you are giving him feeds that heat him, like oats, then that will only make him flightier.
Do you have control of the horse on the ground, or does the horse have control of you. This means is he pushy, does he respect your space, does he kick when you brush him, or pick up his feet, can you leed him or does he leed you, etc.
You may consider finding someone close to you, who knows about horses to re-educate the horse and teach you how to handle him.
He needs to learn that the mounting block is a nice place to be. Take him there when you are not intending on riding and stand there quietly with him. Give him treat and loves. Then take him away from the block and make him move in circles or backwards, whatever, as long as he is not standing and relaxing. Then go back to the block and show him what a nice experience it is there. You are going to have to repeat this as often as posible. Even once you have managed to get on, do the same thing - stand quietly and stroke him.
Finally, check that he is not sore from the saddle or something, he may not want to be ridden because he is in pain.
There is something about the mounting block that is spooking him. As to green and active there is only one cure for this, a lot of wet saddle blankets. He just needs a lot more rideing. After rideing him and wearing him down some lead him to the block. Don't try to mount. Repeat this sveral times till he gets used to the block. He may be spooking at you being above him when you are on the block. This is a natural reaction due to in the wild a horses main preditor is mountain lions and they usually drop from above to their back. It seems to be bred into them. One thing you might try that has worked well for me with mustangs and mules is to sit on the top fence rail and slowly work your animal towards you till they get used to something being above them.
Hey Bolly, Wallis is onto something! I agree, patience darling he's young and you can show him how to be confident with the mounting block by using the method Wallis explained. :) If you are patient enough you can teach him to be a big brave boy, He's not being naughty he's just not confident....You can do this! :)
Try not to be rough or thrash him, he will only learn to resent you...firm but fair and kind will build his confidence in you. :)