How to explain to my boss where not to publish?

I've heard similar tales. The realistic side: you can fret, tell your advisor he should try for a better journal, change advisors, or bring it up as an ethics violation. Yet none of those will get your thesis finished.

You need to concentrate on finishing your thesis. & if that means you need to stop all other projects, then that's what you do. Apparently, you're sidetracking...writing other papers, in addition to your thesis? Or, is it that your advisor has a lot of paper ideas (or you get enthusiastic about new ideas and present the ideas to your advisor?) and you've been very willing to work on more projects? (This can be a double-edged sword...tempting to get involved in several papers/studies, but it keeps you away from your thesis)

You should have a list of journals that are acceptable to you (starting from the best, elite journals to good solid B journals), why are you waiting for your advisor to send YOUR work to a conference or a publication ? Based on your description of the issue, these papers are actually coauthored (with your advisor)- your advisor did the writing, with some of your data.

Now that creates a slightly different problem, -- and it is one you will have to resolve (maybe repeatedly) for your entire career. Some coauthors will send a paper anywhere..because some schools don't care where the paper is presented or published (oh, those schools will prefer elite journals, but at many schools, quantity is more important than quality).

My suggestion: finish the thesis. Don't show any other work to your advisor.