hh stands for 'Hands high'.
A hand is a unique (albeit slightly old fashioned) measurement for horses. One hand is equal to 4inches (10cm). So a horse that is 12hh is 120cm, or 48 inches at the shoulder. A horse that is 12.2hh is 12 and a half hands - so 125cm or 50 inches at the shoulder.
To measure your horse, you measure from the point of withers (their shoulder just at the base of the neck) straight down to the floor. In the UK, a pony is anything less than 14.2hh, a horse is 14.2 hh and above, though this varies depending on location.
The only way to accurately measure your horse is to do it with a measuring stick or tape (preferably with a stick that has Hands marked out on it), though if you post a picture we may be able to give you a rough idea of his height. It won't be very accurate though!