Ok so arguments you may want to use, in no particular order:
He is the one who wants to leave, so he should leave.
Why should you move just because he wants to leave.
You and ds shouldn't be forced to leave your home because of his decisions, it is not your decision.
If you are separated he no longer has a say in where you choose to live.
You have worked out that you can afford to keep the place on your own once he moves out and that is what you want and lan to do.
Until you know what the long term decision is re your relationship you are not happy to move out/sell/rent the place out.
Suggesting you find separate accomodation and renting out the house is basically saying he doesn't see the relationshiphaving a future so if that is the case he should be up front and honest on that. Moving out reduces any scope for a reconciliation from your point of view. So what is the point in relationship councelling exactly if he has already decided it is over...
ds will be unsettled and confused wnough by this. Dad moving out will be more than enough to deal with for now. He doesn't need his who world turned upside down further by moving house, possibly changing schools etc
Home is close to xyz (school/family/work etc) and is therefore convient.
The whole situation is stressful enough without the prospect of moving house. Ds is being affected by the stress and arguing already, you don't want to pile more stress and upset into and already difficult situation for you all.
You will review the situation in 6/12 months once you have spent some time at councelling and he decides what he wants to do (separate/divorce or try and make it work). Maybe things will be clearer by then but while the future has no direction you are not prepared to move.
Ds will get confused if you move out to be separate and then decide to move back in together. So him moving out is one thing, but to also them move him out while you don't know if that is a temporary or permanent thing is not acceptable to you believe this will confuse and upset ds.