Fnf is great but it's a family organisation (not just for fathers).
Ok firstly is there no chance of saving this relationship? He may be a nice guy and work hard and all that, but that doesn't mean he is supportive of his wife and caters for her needs. And it doesn't mean she won't have gone off an had an affair. We can't know what goes on behind closed doors. But if it seems to be lack of communication and is possibly fixable then a relationshipo councellor may be worth a try.
But assuming that has been tried...
It does sound like he's hoping things will resolve and she'll change her mind. But he has to accept that it's unlikely. He needs to face reality and deal with things or he will get shafted re the house and his ds. Are his dc not worth fighting for....
Once they separate he is no longer responsible for her household costs. If she has the child benefit and the children more overnights on average he will pay her maintenance (usually based on CSA calculator here)
secureonline.dwp.gov.uk/csa/v2/en/calculate-maintenance.asp
She will also get WTC and CTC if she works 16+ hours a week plus child benefit and possibly housing benefit and council tax reduction (on top of the single person rate). And that's on top of whatever maintenance he pays her.
So she may not be rolling in it, but it's enough to run a home. If she needs more she can work longer hours.
Re the house. He needs to stay there, in his own room. I know this can be tough, especially when no end is in sight and things are difficult between them, but he needs to do this to protect his rights to the children and a decent house for them when they split.
He really must see a sol about the divorce, particularly splitting of assets. Is this something you can arrange and go with him for the first appointment.
Re the dc, firstly how hard is she going to fight this or is she pretty open to him having the dc regularly? Some parents put the dc's interests above their own feelings and wants, and sum use them as a weapon to get back at their ex for whatever reason.
How envolved is he at the moment? Does he do sch pick up and drop off any times now?
He needs to understand what shared parenting is. It doesn't mean equal time, but it does mean equal responsibilties. He will need to know what this means because many solicitors don't, and many judges need it explained to them too. It is changing slowely, but it's still more common for the main carer to get awarded residency. If he wants it to be different he will have to show why it's better for the dc to have shared parenting arrangegement. Look here for more advice.
home.clara.net/spig/
If possible it is better to agree betwee them re the children. The default position for residency is that neither of them has it unless a court orders it. So sometimes it's best not to force the issue in court.
If they can agree amicably to regaular term time contact (which generally is every other weekend and one week day and half the school holidays - what else was he hoping for? How does that work for him&work, the dc and his ex - He needs to be reasonable and show how this would work and be in the dc's best interests).
Mediation can help with this rather than solicitors. If it ends up in court it can be costly (although he can represent himself (see Fnf)) and lengthy and any court will want to see mediation tried anyhow. So try that route first...
At the end of the day you can only guide and support him. He has to do this and step up to it for the sake of his dc.