If you don't accept any of that, nothing anybody says will convince you. If you want to consider it an alternate timeline, go right ahead. Whatever helps you enjoy the shows (or feel free to watch something else).
I severely doubt him thinking of DSC or SNW as an "alternate timeline" will make him enjoy them. If it wasn't that, he'd find something else.
Note to
@JcarlA: I can't convince you of anything because I only think of the visuals of DSC to be a reboot. The actual events themselves still take place even if they look different. As far as SNW, I think it's more than just a visual reboot. DSC is one level of reboot and SNW is a step above that.
You either like each show on its own terms, or you don't. I still think of ENT as really being more of a prequel to TNG/DS9/VOY than to TOS.
There's TOS Trek, there's TNG Trek, there's Disco Trek, and there's Kelvin Trek. Just think about it like that and be done with it.
"The Prime Timeline" is what's currently in Active Canon. Right now it's a mix of TOS Trek, TNG Trek, and Disco Trek. Things that get pushed out or revised become Inactive Canon or Obsolete Canon. Star Trek Canon is like the US Constitution. It keeps getting amended.
I know some people don't like hearing this but: The sooner you accept that Star Trek Canon has become like canon in comic books, the saner and happier you'll be. That might not have been what it started off as, but that's what it's become over time.
Or the simpler answer, which kids have an easy time with but adults usually don't:
It's all make-believe!