I had a 13hh 19 month old and I'm sure she will make 14hh. Trying to think back to how tall she stood at 8 months...can't remember exactly but I know it wasn't as big as 12hh. Hope that helps?
A very rough guide is 2 hands on the yearling height, so based on that he should make 14.2hh One of mine was 12.2hh at 11 months old and at 18 months old is 14.2hh...
Have you tried taking a string from corner of elbow down to fetlock joint, then hold it at elbow, and bring the other end up - that's the old fashioned way of telling where the wither will come too when grown. Think it pretty much worked for mine - he might have been an inch or two taller.