Where is your father and contribution from his side? It sounds like this is a bad situation that's been lasting for a long time. Your stepfather wants to contribute to his son, but your mom keeps objecting and all the resources go to you. His sadness is understandable, but it's also something they should've figured out about a decade ago. This shouldn't be your problem. You're not wrong to want to keep your savings for you. Your stepdad put you in a shitty position by asking you about it like this, as you haven't been part of the lifechoices your parents made.

See if you can get a bank account of your own, without any parent on it. When you're 18, you should move out asap. They're not going to solve it any time soon and in your stepdads eyes, you're going to be the one to blame if the relationship with his son stays shitty. It's easier than blaming himself.