I think you will have a better estimate of your young ponies height at the end of the season–as in late fall. IME they tend to have growth spurts both in spring and fall till the end of their 3yo season. After that its mainly wither height/body depth/muscling that changes—most often the front tends to catch the hind end.
I just measured my two year old purebred with a regulation stick and at 2y 3m he stands 14HH at the wither. I expect him to most likely remain Pony Height (14.1-14.2). His mother was a maiden and he was a smaller foal at birth. He is roughly just between the heights of both sire and dam at the same age—I raised all three of them and tracked the growth on all three from weanlings on up–so I have a very good idea on how each grew. Mom is 7 and 14.2 1/2 and dad is 9 and 14.1 (possibly slightly more but we are talking very small increments-cm maybe).
I string tested my youngster this morning–by the Ergot to Elbow method he will finish @ 14.3+HH. By the center of the knee to coronary band he will finish closer to 14.1 which is closer to what Im predicting based on more practical observation–though Im guessing closer to 14.2HH when his wither comes up. This colt happened to have a prominent wither as a foal–so I do expect him to finish more ‘uphill’ than is typical for the breed. I personally would be delighted if he made 14.3±–but Im not going to hold my breath either.