I think his Friendship points just need to be high enough. I don't know what the threshold is. But he won't do anything before he trusts you.
Is there a way to repair the friendship? I took the Garth route and I haven't seen a way to interact with him since.
The Garth route doesn't prevent interactions with Egon, so it must have been something else you did.
I think it's because I rejected him at first and then taking the Garth route just lowered the friendship points even more.
