Cast is where a horse rolls and cannot get back up they are stuck thats why you need enough room in your stable for them to lie down, role etc and have good banks as they may help stop them getting cast.

 You will find them way too small i have a 14.2 in one a lot bigger than that.

The minimum size stable for a 15hh horse upwards is

12x12ft, as this gives them enough room to prevent them from getting cast!

The bigger the horse, the bigger the stable needs to be!

But if its only for short periods of time he should be ok!