I got ED-E to listen to all of the voices from people in the Wasteland. Then I took him to The Followers of the Apocalypse to get his upgrade. But after that, whenever we get into combat, he just sits there not moving or attacking anything. Even if I kill all of the enemies, he still won't budge until I fast travel or enter a new area. What happened?

thats one of the many bugs maybe try loading older save and not finishing that quest happend to me as well :facepalm:

did you happen to take him to a casino after the upgrade? theres a glitch where ed-e's weapons aren't re-enabled after leaving a no weapon zone

i'm soo handing him over to the BoS since it doesnt affect the ending anyways (*does actually )plus i bet it's the weapon upgrade the specifically causes this problem.