I really don't have any idea how genetics like this work out. I think it's all a matter of chance, really. It seems to me that the foal shouldn't be much shorter than its dam if any. At the very least, he might be an inch or two less that his dam. At the most, maybe an inch or two taller than his sire. Or, I could be completely wrong and he'll grow to be 17.1 hh... but I sort of doubt it.
Alot will depend not only on genetics but proper diet as to how tall it will get. Odds are good its not going to be alot different from the sire and dam by 5 years of age but I've seen cases where without proper diet they never get close to their potential height. And I've had a few that because of past generations as iridehorses mentioned can get much larger. I had a stud that's dam was 14.3 and sire 14.2 as a 3 year old he was 14.3 when he was 5 he was up to 15.2. But he had granget that were over 15 hands.