Well, I would hope that instead of "telling" him, you would sit down with him and explain that this is an important issue to you; could some scheduling changes be made to accommodate your beliefs.
But the other thing to ask is whether your job is a work of mercy or necessity. I reference the WCF, chapter 21:
VIII. This Sabbath is to be kept holy unto the Lord when men, after a due preparing of their hearts, and ordering of their common affairs beforehand, do not only observe an holy rest all the day from their own works, words, and thoughts about their wordly employments and recreations,[38] but also are taken up the whole time in the public and private exercises of His worship, and in the duties of necessity and mercy.
Are you a doctor or nurse? Hospital orderly? Ambulance driver? Policeman? Fireman? Do you work in a nursing home? If your job is a work that is
necessary and/or
merciful, then it would not be a violation of the Sabbath, but rather obedience to that commandment. In such a case, it would still be expedient for you to arrange your schedule so that you can regularly attend public worship on the Lord's Day. If it turns out that you still must work one or two Lord's Days per month, although you may wish to rather be at church, it is not a Sabbath violation
per se if your work is necessary.
If you work in sales, then, yes, you should make every effort to completely cease working on the Lord's Day, for all time. Yes, this can seem extreme in today's cultural environment, but this is what the commandment says, and you would be doing it out of obedience to and love for your Heavenly Father. Will the Lord not bless you with alternative arrangements if you diligently seek such a change?