Of course you can respond when he files the separation. Lots of things depend on the state you live in. Start looking at online legal resources for your state. Why do you think he is filing for separation and not divorce?
M:38 H:42 T:20 M:19 D:18 S:17 MLC: Sometime in 2007 OW Bomb 1: 12/28/07 OW Online relationship 2: Spring 2009 Told him I wanted D: May 10, 2009 D final: 07/09/11