And also, is he expecting you to split your CTC and Child benefit in half? Or just the bit that you get for your DS? Which will actually be quite a bit LESS than half of what you get, as you have an older child.
With your CTC, you will get a 'family element' that is paid because you have a child or children. That part is classed as not for either child individually, but for family expenses. Then you will get two child elements. One is for your DD, and should not be split, it is all for her. Only the remaining portion is meant to be split for your DS, morally. The family element is to cover the fact that you need larger housing with higher costs when you have the main residence for that child, and also to cover additional things like the extra electricity used etc. this was all explained to me in court because my ex was doing what yours is trying, but actually went as far as taking me to court over it.
Child Benefit - you actually get MORE than half of your Child Benefit for your DD, and LESS than half of it for your DS, because he isn't your eldest child, your DD is. So only the part that is paid in respect of your DS should be considered in a split of Child Benefit, otherwise you are giving money paid in respect of your DD to your useless lump of an ex. She shouldn't lose out.
And be VERY careful whenever you give an ex some of your CTC or Child Benefit to state quite clearly, every time, that you are NOT setting a precedent by doing so.