That does suck, and Im sure you've already talked, but I'd say dont be mad at him for telling you over email, that's a hard thing to tell somebody, and I'd focus on the upside of him wanting to tell you right away.
Overreacting? I would feel hurt if a fluid bonded partner chose to fluid bond with a new person and go back to using condoms with me if they had problems performing with them. I'd imagine it was because people get scared of performance issues with new partners but he feels comfortable with you so feels more at ease having problems in front of you? So I'd understand, but I'd be pretty unhappy too, but I'd try not to look at is as having them choose somebody else OVER me.
Happiness will never come to those who fail to appreciate what they already have.