Your boss is an idiot. And not doing his job.
Part of his job is making sure that your workload is reasonable. Also part of his job is appropriately managing all employees who report to him. He's doing neither.
If I have someone helping me with work, then yes, I'm training and providing some supervision, but if they mess up it's not ultimately my fault. They messed up. Now, I may have messed up as well if I didn't identify the mistake and I was supposed to, but that's a different issue.