Wow. A really complicated situation.
For starters, what jumps out is, the eldest is at Uni. So he doesn't actually qualify for "child support" in the respect of what the CSA are used for. Any money your partner pays to him (which should be direct to him, not the Mum really, at his age) is voluntary.
So that would decrease the figure squashedfrogs came up with as there are technically only 2 qualifying children, to £61 a week for the 2 youngest.
The problem is, as much as it's grating on you, will your partner do anything about it? Will he be worried, making changes will rock the boat? But then your child between the two of you shouldn't be losing out and he needs to remember that.
He has two choices that I can see.
Approach the mothers and offer an amount he can better afford, if he can afford to pay more than the CSA would ask for, informing them of how much you would pay through the CSA so they know they are actually getting more than the minimum. They shouldn't get different amounts either really. They should get the same. (or if you want to complicate things further, the Mother of the youngest should get slightly less as on average it would appear you have the child overnight for 2 nights a week).
If they don't like it, contact the CSA and ask for an assessment from them. It looks like they would assess you at £61 a week ...so each mother would get £30.50 a week for their child.
Offer to start paying that to them direct or if they are funny about it, put it away in another account that wont get spent, while sorting out what to do, so you know you have provided for them somehow.
When speaking to the CSA, you should also ask how they work things out when there are more than 2 Parent With Cares, and one of the children stays more than the other, because the mother of the youngest would get less than £30.50 a week due to the child staying with you overnight regularly ...
then look at it another way and maybe it would be better just to say forget the over nights as sadly, when some PWCs notice a reduction is given for it, they stop them and the child loses out.
Hope that's given you some food for thought.
Sorry it's long.