maybe him threatening to sue was a quick defensive action to throw you off from thinking you can do the same as you may now just panic about a possible big law suit and not how you can sue him.
is anyone stll in the house? if you're serious about taking action against him, or even serious about others lifes then i'd get everyone out immediately. house could be unsafe and ready to fall down any minute.
don't sue if it's not needed, if you're ok other just a few cuts etc then get on with your life, don't ruin his for a quick bit of cash. i'm sure he wasn't aware the floor was about to fall through yet still let you walk around on it. he sounds like a guy who is unsure what he's doing as it's a crazy place to be, knowing the house you own with people in it is falling apart people tend to act a little weird. give the landlord the benefit of the doubt before calling him an idiot people.
when did this happen? part of me is thinking the op and friends are probably students, and probably cause all sorts of problems for the landlord, and could possibly have come home from a saturday night out really drunk, messing around, and then jumped from the bath onto the floor, resulting in the guy falling through it.