At 7 months of age they are "supposed" to be about 86% of their final height, which would make him due to get to about 15:2 or 15:3 (54 inches * 100/86).
Or, you can measure from the middle of the side of the knee down to the coronet band. The height in inches is "supposed" to be the final height. e.g. 15 1/2 inches would mean 15:2.
Use of quotes though because I am not vouching for the accuracy of either method!!