This is a very immature horse for a 3 year old. He has a steep shoulder and has an upside down neck. He tends to carry his head high and his back hollow. I have attached his photo with the upside down neck.. over developed neck muscle outlined.
He is light boned and tied in at the knee. He is sickle hocked and.. one of the few times I can say this.. Cow Hocked. Most horses toe out behind as the hind leg is a spiral and functions like a spring. To have the stifle clear the belly, the hind toes must point out a bit. This horse actually has an angle of deflection at the hock. He has a nice roomy hock which will help him.
I like that he gaits and had a good attitude but he is a pretty light horse for the rider on his back. I assume he is not a big horse.. maybe 14.2 hh maybe coming 15hh... and he is very immature looking. I think I would back off and let him mature more. I would help him by getting long lines on him and doing a lot of ground driving up hills (think of the shape you will be in LOL). This I would do at least 4 days a week.. to try to get him to develop abdominal muscles and get that neck to turn over.. and to teach him to lower his head. He cannot do that w/o building the right muscles to support himself.
He really needs to bulk up.. if he will. Some never do. I remember a couple of horses I saw every day for about 20 years that looked forever like this horse (immature.. almost like yearlings). However, you can help him as much as possible and see where it goes.
He is an interesting color. Do keep up on his feet.. this will also help him a lot.
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.