I think ex(D)H should be in self isolation?

[WishIWasAnAlien]

I was very ill last week, GP telephone consultation they beleive i had Coronavirus. I co-parent and DS stays with his Dad twice a week including after schools and half the weekend. DS is in isolation until Sunday, though I am now out. I presumed ex was in isolation because of DS, however now I need to return to work tomorrow he is saying he is in work and will continue going in. He is not a keyworker. I am.

Should he be self-isolating?

You are two different households. So your ex would only need to isolate if DS had symptoms.

I think he probably should have been i isolation since you developed symptoms. If DS carried the virus (and could be asymptomatic) to exHs household, they would then carry it to wherever they went.