So, when I got the OK to have Rex follow me to Jacobstown - I decided to hold off on that quest. I'm playing hardcoe Mode and didn't want him to immediately die and leave quests, etc. unavailable like what happened when I got Boone following me.
So, I tell Rex to wait right outside The Kings' headquarters in Freeside, but all around Freeside people will talk to Rex when I approach them alone and he's waiting on the other side of town.
Just sayin.