You need to act as if you’ve already separated, go out with your friends, act as is if he isn’t there, don’t do anything for him.
Move into the spare room if you have one or buy twin duvets, don’t ask him any more questions about why he wants to separate, do not make his life easier in any way.
Do not have sec with him, if he’s met someone new he could be all fired up and want sex with you too.
See a solicitor, decide what you’d like to do regarding housing/finances etc, don’t let him decide and dictate terms to you.
Ignore the word separate and act as if he’s asked for a divorce. By separation he could mean keep things as they are and then he comes and goes as he pleases.
Tell whoever you want in real
life, don’t keep it as some dirty secret, get it out there, your DH wants to split up your family.