Yes, if his work is suffering due to his exhaustion, then his workload is too much, but you can't change that. All you can do is tell him that you aren't happy with the job he did, tired or no.
When my brother has a lot of horses to do in a day, the only way to tell he's getting tired is that he'll start going slower. He takes the time he needs to get everything just right. He might get the first horse shod in 20 minutes but the horse at the end of the day might take an hour.
Perhaps because he's young, he just hasn't learned to pace himself yet. It may take him a big of time to figure out that the job you do is much more important than the time it takes you to do it.