I have been where you are now and I can offer how I dealt with it in case it may help you -
I realised the following - I was bearing an emotional weight that I was never meant to bear. Everyone has a first responsibility to themselves unless they have a young child and then it is the child and then themselves (as I'm sure you know!) You have 50% responsibility to your child in every way and you decide how much of this responsibility you will undertake to fulfil - Like-wise your child's father decides how much of his 50% responsibility he will fulfil - You cannot make him fulfil any of it no matter what you try.
Like Jen-Mum say's when your little boy is older he will see each of his parent's for what they are, when they were there etc..
I let go, It is not my responsibility to try to get another adult to fulfil their responsibilities. I never tell people what to do but I do sometimes tell him what I expect. Sometimes he say's he doesn't have the funds to pay maintenance this month and I say "Well, you have your responsibilities to your child to do so - If you don't pay, I am not going to beg you - It's up to you and your own conscience" and then I carry on, he walks round the block to have a think, returns and promises payment.
Another thing I did was to refuse to talk about our relationship and I always bring the subject back to the child because that is what his involvement is. He say's "Here's your money" I say "Oh, you mean X's money - Thank-you"
You were never meant to carry the weight of his responsibilities on your shoulders. Let go, Don't cut him off just let him do or don't do and you will find that your child will take it in a matter of fact way. If he say's "Why is Daddy not here?" say "I don't know but why don't we do........."
Hope that helps