Horse getting tired and running out of energy to soon?

My Thoroughbred horse is getting tired and running out of energy to soon in the race. In the last turn he doesn't have any more stamina to finish the race. Last year he was doing really great and this year he started to slow down a bit. It's probably the trainer that isn't doing there job as good as last year. So is there any other thing that I could do to help him get faster and last longer. Any supplement, work-out routine, or any option that will get the horse that extra help to finish the race. I live in Texas and take the horse down to the Lone Star Park in Grand Prairie.