Ok, so I think that we can all agree that the Gen1 synths are entirely artificial androids created to roughly appear in the base, human shape.
Gen2's, also pretty much synthetic androids, just a little bit more improved.
Gen3's... that's where we kind of tend to disagree.
Then, there is Nick Valentine and Dima.
Gen3's are, in MY mind, artificially created human bodies, created from cloned genetic material. They are then implanted with limited cybernetics, mostly in the brain. (Was Kellog a test rat for this procedure?)
Nick V. Nick I consider to be a gen 2.5 prototype. I see him as somewhere there in the line where they started to get results in merging some of the organics (brain tissue and memories) into the inorganic bodies.
Dima I see as a gen 2.75 (ish) prototype. They've successfully integrated the inorganics into him and have successfully created a synth that is capable of independant thought, and completely capable of creating it's own personality and identity.
Kellogg - Ok. So no, I don't feel as if Kellogg himself is a synth. However, I do feel as tho it's possible that some of those cybernetics that the Institute gave him, did use him as a test subject for getting the "man and machine" to properly interface.
It would kinda fit along in the timeline. He is, after all, past 70-80 years old.