I'd take him out, QT him, and continue to cycle the tank fishlessly with pure ammonia. That's what we did. I purchased mine from Ace Hardware. Must be 100% and double check by shaking it and see if it bubbles. If it does, then don't buy it. Truly pure won't lather.
There are calculators out there to determine how much to use, but I can tell you, it's a super small amount. Like, I only used 2 tsp I think to cycle mine!