Sure you can stand on any horse ;) the only question is how good your balance is and how long you'll be standing. LOL
He's cute. He has his ears back in all of the pictures, though that may not mean anything, but what worries me is that he is in a Mechanical Hackamore. Is he even bit trained? Mechanical Hacks are very, very, very strong bridles to use on a horse and might be either a sign that he isnt fully trained, he isnt good with bits and throws a fit, or he's very 'strong' when being ridden. Its also incorrectly fitted.