I have the same breed of horse, and around the same age he would stop and stare at things out on rides and you couldnt get him to move at all. It was his little baby brain processing things as fast as he could, just not as fast as his legs should be going. If I just sat there for a minute his brain would catch up and he would carry on. Some days it was really bad (as in lots of stops) but he grew out of it, and it wasnt worth picking a fight over, because thats adding more stress for him, and you never got him to move until his brain caught up anyway.
Have you tried ground driving? If he doesnāt feel that he can carry a rider because of his lack of coordination, that might be easier on him.
I totally agree with this!
Agree with age size appropriate. Still need to get a sport vet to listen and deal with your concerns. Rood n Riddle or New Bolton?
Checking out physical issues makes sense but honestly a lot of larger young horses arenāt ready to do much work until 5. My homebred oldenburg mare was big and a bit slow growing, so we did a lot of groundwork but waited to start her under saddle until she was almost four, but then took it easy the following year. Last year she would try to stop and plant her feet, so we did fairly short sessions in the ring and hacked out following other horses. Now sheās five and much more balanced. Sheās a pleasure to ride, and fine going out cross country.