Question for step moms

I have a 5 year old stepson (we aren't married yet) but since about 6 months into our relationship I have bought all of the shoes and clothing for him. Almost all of the stuff I have bought for the past 3 years his ex has taken, resold or thrown away. We are expecting our first child together soon. I've pretty much stopped buying my stepson stuff

1. He has a lot of stuff that still fits him

2. I can't afford to pay for everything with this baby from wipes down to insurance.

So my question is how much do you help out with your step/extra kids? Am I in the wrong?

Also this all got brought about because she threw away his only pair of shoes for our home. I told him I'd buy more once I got paid. But I added "even if I don't think I should have to keep buying things for him when no one appreciates it and it gets taken away"