Thanks for the detailed input, I appreciate it. He is actually 15.3 hh and is expected to reach near 17 at full maturity. I'm not sure how familiar you are with the breed, his head set is what's desired in a TWH, it's not carried low like a QH. He has filled out significantly since this time last year. He is kept strictly on lush hilly pasture and brought in and grained once a day.
I'm hoping he will bulk up more, but it's not likely given his build. Many TWH are lean built, much unlike a stocky rotund QH lol.
I am familiar with the breed. Long and lank is different than immature build.
Have you had a stick on him? At 3 it is unlikely he will go 17hh ever. Height growth is usually finished by 3 coming 4. By 4 it is done.. unless the horse is under nourished (and then if it is bad enough the growth may be stunted forever.. or start again with malformations).
FWIW I bought a stick when I was in the horse business. One of the best investments I ever made. I still have it (though not had a horse for a number of years). I always brought it with me to look at a horse 4 sale (and do to this day when I go with someone else). I get the stick out and it is AMAZING the 16hh horses that are suddenly 15.1hh. In the world of English riding, a 16hh horse brought more money so I sticked every one... and the prices would change because the seller could not argue with the stick. LOL
Pasture and grain will NEVER build a horse up. Hay and grain will.
A horse can have a relatively high head set.. but still should not have an upside down neck. With a higher head set, you still want support from the root of the neck which this horse is unable to exhibit.