My 14 month TB colt seems to be pretty short for a yearling?

His mom is 16.2 and his dad is 16.2, his full brother matured to be 16 hands. Ringo is very small, even the vet said he was the size of a 6 month colt even though he was 12 months old. When i do the string test it says he is going to be 16.2 but I don't see it. He is a stud colt, 14 months old and had knee surgery 2 months ago. He is just now developing his chest and neck but hasn't seemed to of grown more than an inch in 2 months. I'm afraid he is going to be very small but I guess I'm wondering if anyone has had their yearling to develop later in life and grow to meet his full potential?