I want to join the Atom Cats, and I've looked all over the map in the area the wiki recommends to no avail. I have seen posts mentioning that he's been seen in other locations, so just how big is his spawn area? Can I go directly to the garage and join that way?

no one has any input?

I bumped into him fighting a Deathclaw Alpha, near some old cars a 100 or so metres Northish from a Raider camp by a big stone tower with an air raid siren on top of it, a short way north of Saugus (spelling) steel works (where the founders are).

From the dialogue he appears to be searching the cars for salvageable components.

I was doing the Cabot questline at the time, it was a short way to the west of the location you go to retrieve the serum.