**Don't impersonate your manager, even if he asks you.**

Ask your manager *why* he would prefer you take the test instead of him, and offer to help address that *problem*. (Treat the disease, not the symptoms.) If he finds the material too difficult to understand (perhaps because he doesn't have the relevant background), then offer to teach him or provide him self-study material. If he doesn't find the time to study, offer to temporarily take up some of his less critical responsibilities. 

Most companies have a policy against this kind of impersonation, and silly as it might sound, your manager could be doing a "sting operation", as part of a compliance auditing. If you agree to impersonate him, he has evidence against you!

Even if this isn't a sting operation, this impersonation is not only unethical but could also create problems. For instance, suppose your manager is deputed to work for a client who requires a person to have completed this certification, and they later find out that he did not take this test. If things turn ugly later, you don't want to get muddied.