If you plan to turn down the offer from the new company, why do you apply for the job at the first place? Sounds irrational (since they would not consider your application next time). Anyway, if you could find a reference other than your boss, it would be great. However, some employers ask for (and check) references at least one from your current boss directly before they give you an offer.
On the other hand, if your current boss knew you are going to jump out to other boats (sooner or later), he/she might not promote you anymore, unless you are the critical staff there.