How much more will this horse grow?

There is this beautiful grey horse at the barn i ride at, his name is Falcon and i ride him once a week. They are looking for someone to buy him next year because his training started not too long ago and he is still green. He is 4 years old and 17.3hh, i was thinking maybe he will grow to about 18.2hh more or less. What do you guys think? He is a ThoroughbredxHanoverian. Please answer, thank you :D