I don't remember what came first, the egg, or- I meant finding out about his mother being thought to be in a dangerous lolocation or Eris abandoning him after the night they had first had sex together.
Either way, his ED started when Eris abandoned him, and I suspect the reason is that the abandonment made him imagine that he wasn't good enough at sex for Eris, and that's why she decided to leave him.
On top of having constant performance anxiety because of that, he also had to deal with the abandonment itself, how raw and painful it must have felt for him even if they hadn't had sex the night before, and all the thoughts of what he did wrong aside from the thought of having performed inadequately in bed.
Then you have his family members who are still missing somewhere, having sex with someone with that going on might have caused him to think what he was doing was wrong, that he shouldn't be having fun and sex while, for all he knows, his remaining lost family members may be out there all alone, in pain if not dead.
Those things combined would definitely be a recipe for ED in someone.