How do you know your mom provides more than half of the support of her boyfriend’s children if you don’t know what he earns? If he’s providing health insurance for them that’s a decent amount of money.
When you say your mom pays the “majority of bills/food,” what does that mean? Does she pay for utilities and food and he pays for rent? Or is he not paying anything? If he works, he should be supporting his children. If he can pay when your mom runs low on funds, maybe she should get him to contribute all the time so she has money to help you pay for school.