Man, the poor guy must be petrified by now.
Just to ease his worries a little - the first stage is just the C&D. That's just a letter asking you to take it down, though it might be strongly worded. Generally speaking, if you comply it ends there. Even I've had one in the past from WotC legal (due to stuff folks had posted here), though I had a lot of contact with WotC back then and it was all very amicably dealt with.
Following that (if you don't comply) would then usually be a takedown notice. That gets it taken down for you.
Anyway, you've taken it down. Don't panic too much. If you do try this again (though John Reyst makes his living doing exactly that and has one in the works, shocking exactly nobody - and even he has made mistakes and had requests to remove stuff from the likes of Paizo) it's worth getting legal advice. Costs a bit upfront, but peace of mind is worth it.