SW - I wonder if part of your confusion stems from not having properly separated IYSWIM.
If I have this right, he comes round most days to help with the DCs and TBH sounds like a fully fledged member of the family, except that he does not have staying over rights, and that his time with the family are controlled by you, rather than in a full partnership with him.
It hasn;t been a typical split where you manage almost a complete fortnight on your own and then hand the children over for a couple of days of dad time.
I understand fully why that is the case, you felt that your DCs needed to see their Dad more often than that.
Nevertheless, it does sound confusing for everyone concerned and perhpas DS has not had a chance to get used to a new routine with set times for seeing dad.
I agree that it might be best to go back to relate. It might also be worth seeing whether there are anger management courses that both of you can go on. Only you know the path you are willing to take, but as things stand the risks of it going belly up again do sound quite high to me. Perhaps a proper separation is needed before things can start to be rebuilt again IYSWIM?