I want my fiancé to take a DNA test

Long story short, my fiance and I have been together for 4 years, engaged for 6 months. His son is 6 years old...him and his childs mother co parent peacefully and he is a very involved active father.

Since we began dating, I never thought his son looked like him...but to be honest...he looks just like his mother...I DO NOT think, that is his son. He looks nothing him or any one in his family.

How do I go about doing a DNA test? Should I tell my fiance or should I do the test behind his back. Or should I keep this to myself.

I know if my fiance were to ever find out his son wasn't is, he would be absolutely devastated.

But I cant seem to shake this feeling....my intuition is telling me he is not the father.