I- 485 not K3!
K3 application is a nonimmigrant visa petition which is made, or can only be made rather, at a point outside of the United States in order to admit the alien spouse of a U.S. Citizen while waiting for the result of their I-30 (Alien Relative) application. It really does not make any sense for you to get married and for him to then return to Canada in order to apply for a K3 Visa.
You are both here in the U.S. and the most effective and logical way to go about this is once you are married, for him to apply for an Adjustment of Status (I-485) which WILL affect his work permit and visa for certain, but not in the negative way you might think. In fact, once you are married it is not going to matter whether he has a work permit or not as in such instances the applicant is issued an automatic (temporary) work permit immediately upon filing of I-485. In your boyfriend's case, his current non-immigrant work visa and permit will be superseded by the temporary work permit.
By being married to you, he will of course become an Lawful Permanent Resident (L.P.R.) and a Green Card holder either way, but the time difference between applying from the outside as opposed to from the inside could very well be 1 year, 2 years, and may be even longer.
fredrikklaw