OP,
I mean it kindly but you are very foolish to be tolerating this.
He is using you.
He sounds like a shit, selfish husband and a shit father to your child.
You are subsidising this man financially, when you should be saving up for a running away fund.
Tell him NOW that he needs to pay for HIS family's gifts, you will not be doing it.
And mean it.
Why would you want to be with someone who doesn't care about his child to buy them a gift?
Stop stepping in and making everything ok.
Let HIM be embarrassed.
Tell his family that HE is doing the present shopping this year.
It will be on him.
This will never change until you force it.
You are just another woman that was roped in by a waster to be au pair, house skivvy and bank roller to his children.
They invariably are less invested in the child they have agreed to, so that you feel stuck.
You are not stuck.
Do not settle for this.
Keep your money and start saving.
This situation is only going to get so much worse with this waster.
The teenage years are so expensive.
YANBU 