Is my position being undermined if my boss delegates work to my subordinates?

It really depends on the cirstances. If the boss has a project he wants done, he can give it to anyone he wants. Unless his actions constitute a a violation of specific company policy, "reporting him" is not going to get any response you could want. What you should do is talk to him in a reasonaable and professional manner that you take your managerial responsibilities seriously, and you find it a difficult to supervise and manage staff if they are doing projects you are not aware of or involved in, and ask him to keep you in the loop, or go through you. That's a reasonable request, and his response will reveal whether or not he is trying to undermine you or is just doing things his way.