I think your idea of waiting until he has a place to live is wise. That way there will be a concrete plan and a specific new home for their dad that they can see, so they're not left with a bunch of unknowns. Depending on the situation, if you are both able to co-parent peacefully saying something right before he packs and moves might work, so that they have time/access to ask you both questions, and also so that the actual move isn't a surprise or happening without explanation.