my son filed his tax return in Feb, this year. He failed to check the box that says someone else can claim him as a dependent. I need to claim him as he lived with us the entire year, and we supported him. What to do?

If you claim him on your return, it is likely that an e-file will be rejected by the IRS because he was claimed as a dependent on someone else's return, (his own).

 

If that happens, you will have to print and mail your return.  

 

You will receive correspondence from the IRS to resolve the conflict between returns.

 

Another possibility is that you could e-file without claiming him, and have him file an amended return not claiming himself.

 

After both of those returns have processed, you could the file an amended return claiming him.