Why does it say an amendment 1040-x can’t be done until March 25th, the IRS’ website says it can be done now. Accidentally claimed my son & it’s messing up his taxes.

The software version of the 1040X will not be available until March 26.   if you want to use the paper 1040X and prepare it by hand so that you can mail it in then you can do that.   Otherwise, if you want software to help you complete the form, you have to wait for it.

As to messing up your son's tax return----he has to file on paper anyway.   Even if you amend, his SSN is in the system for this year and your amended return will not fix that.   He cannot e-file this year.   His return will have to be printed, signed and mailed.  He has no reason to wait for you to amend.