Dependents

I tried to file our 2019 taxes electronically today.  My return was rejected over & over. After some investigation, I found out my husband's son who was 17 in 2019 and was 100% under our financial umbrella; filed his taxes and stated no one else can claim him as a dependent.  Under no circumstances did my husband or I agree to this.

Now I can't file our taxes electronically without removing my step son as a dependent. This will cost us an additional $500 in taxes on top of what we owe.

How do I rectify this?