Lied, or just never discussed?
Whatever happens here, I think it's a case of some very blurred lines. It's the constant chat element that makes it messy. If you get through this bumpy patch with some kind of agreed way forward, it would be wise to start to take steps away from the level of contact, as I think it's propping you up emotionally, more than you realised - maybe that's on his mind too. It's not sustainable, or certainly not in a casual relationship.
When establishing my first FWB he was very chatty, and I remember thinking - this isn't what I wanted, it is nice though, but how do we climb down from this level of chat - so I talked to him about it.
Its now settled, we check in sometimes in the week here or there, talk about various life events (the Friends part) then arrange to meet (the Benefits) and the chat might amp up a bit in anticipation of it.
I have also spent time wondering how it will end - I expect it to fizzle out. Or one of us to find someone else. I'd like to retain the friendship, but we're not that compatible (aside from great sexual chemistry) so maybe that would fizzle out too. I'd not like him to drop off the face of the planet though. I'm one for closure.
Anyone going into a casual relationship, in whatever form you agree upon, it's important to consider some of this. Even if it serves to remind you that it's casual, it's not built to last.