what gives? I go to tell him about the stealth boys and he immediately attacks me. I restarted many times, all same result. Now I had to kill him and take that huge bumper sword. what gives? DO I lose out if I kill him? DO I get a perk from him being around if its possible to keep him from attacking me?

There really doesn't seem to be a downside from killing him. Davidson and the Nightkin he's with are part of the "State of Utolbitha", a bunch of human-hostile super mutants on Black Mountain. So you'd really be doing a "good" thing by killing him.

If for whatever reason you'd rather NOT, then selected Davidson in the console and type "disable" (enter) followed by "enable" to make him friendly again. Should fix the glitch.