With proper work he will lose that ewe neck, gain a top line, have a stronger hind end, and basically gain muscle all over, but there still are the other comformational issues pointed out by the other posters. If your planning on showing soon then pass. He's not physically ready at all, so I would only consider him if you plan on spend lots of time training and working with him. You may not even ride him for awhile, it would probably be better to get him more in shape first by lungening and doing lots of ground work with him before you do under saddle work. Even if he is 'well broke' it doesn't look like he's been worked in awhile.
^^ I can't see it either..I will say that I haven't ever heard a bad conformation critique by SpeedRacer, AlwaysBehind, and equiniphile.. and the fact that there is no completely fixing an ewe neck. Like equiniphile said, it's a defect. You can't just expect muscle to fill in that area. I can't really say much more because I can't see the picture..
He looks just like my boy. Mine is also a TB off track rescue. He is 9 years old. I love him, but I am fortunate and he is a good horse. Some are just plum crazy off track though. My uncle has two and they are crazy mean. Spend a few hours with him and really get into him before you make a decision. That's what I did. Get him without his current handler and just take him alone to see how he will act.