It is done at the highest point on the withers.
Visit this site, It can help you accuratly messaure him. Measure Horse Height Accurately
His dam Holey San Gold, is bred just about the exact same as one of my mares and she is 14.3 - she is about 9, and done growing of course, but she is very short, and we recently bred her to a 16h stud bred sonny dee bar and while her half brothers are 16h+ our filly is quiet short. Considering this mare is bred from lines of cutting horses, its most likely they were smaller, and she was smaller, your gelding may stay quiet small, for being cow/reining bred.
On the stud, alot of poco bueno horses seem to be either short or huge, my 5 year old is poco bueno bred, and she is just about 16h. Sadly! I wish she were a bit smaller.
What are you planning on doing with him?? I would just give it some time, Coming into a 4 year old he's going to be still growing upward. Usually by 5 they start growing width wise.