I can imagine your discomfort in this situation. While I would at first have suggested having his email as the reply-to address (or have postmaster forward bounces back to him), you did say you wished to retain your job, so I won't suggest that particular solution. For this reason, I also will not suggest slowly sabotaging or causing the program to slow for the same reason.

One thing you may do is warn him and his superiors of the potential reprocussions, such as the fact that users in several states have the right to take you to court or sue your company for spamming and the potential costs involved in defending/answering these cases. You might also advise them that even though they currently have the server through XYZ big server company, if they get enough complaints or the emails cause them enough problems (such as causing their allocated blocks of IPs to be blocked, possibly resulting in impact to them or to other customers), they still have the right to decline to provide service to your company.

It is my hope that other monks herein can provide other helpful suggestions. I sincerely wish you luck in finding a solution, and I hope if you do that you will post a follow-up telling the solution you choose.

Update: I could not recall the articles I had seen recently regarding lawsuits and spam. I searched and found several of them (warning: outside links), including references to lawsuits and legislation in Utah (also here and here), Ohio, Washington, and California, the economics of spamming, why spamming doesn't work as expected, suits by the FTC, and spammers being sued to stop using a particular service, among others.