Deductions & credits

I’m 100 sure I checked the box because I got confused on how I was able to get the EIC. He is trying to efile through TT and it’s not letting him saying the ssn has been claimed already. He did not click any of the questions correct about court papers or anything. I am the custodial parent and he only goes to his fathers every other weekend. I tried to explain to him that by IRS laws he is not allowed to claim EIC due to our son not living with him more than 6 months. Our court decree states he can claim him as a dependent.  Do I need to do anything more?