I think one thing you have over looked is that you are assuming your ExHs maintenance is guaranteed. What if he has an accident or becomes ill and stops working? What if he is made redundant? What if he has more children and it reduces? What if he dies?
I think you need to keep saving it in case this does happen, or you become ill, or made redundant as it seems unlikely to me that your DP will be happy to support your DS.
Have you thought onto university? I'm assuming that the student finance system will be broadly the same as it is now, so in that case your DPs income will be considered as household income for grant purposes etc. Will he be happy to support your DS through university, as I believe that if his income is considered, he should? If not then the savings could support DS through university if your DP refuses.
As mentioned upthread, your pension contributions do need to be increased out of the joint pot, if your hours and income reduces due to having his baby. At least until you are married and have a claim on his.
It isn't romantic, but you need to be practical about these things. Women always come off worse in these type of situations.
FWIW - I think what your ex pays towards your DC is none of his business, the same as what he pays to his is none of yours. Your ex could stop paying anything and as long as you can support your DS it is not as if you are asking him to. He is getting the better deal here. His living expenses are lower living with you, he has you to help and support him with his kids, share the expenses of his kids when they are there, cook, clean for and entertain his kids - he wouldn't have that as a single man.
If this is causing trouble now - it doesn't look good.
If you are buying a house together, have you discussed inheritance and wills. How would the house be spilt when you both pass on? If anything happened to you, how would you share for your DS be protected (as in trust etc), he could get re married and then DS gets nothing. In short how much do you trust him to do right by DS as if he is after his money now, I'm getting an uneasy feeling.