Someone has been using my son's social for the past 2yrs. I think I know who it is I've filled out the form the IRS sent me and now I want to know what can I do to prevent them from using his social this year?

You can not, You can put a lock on their credit reports though the agency, but you will have to go though this every year if someone claims them first.

Is it merely someone claiming them, or do they show income from them.

What do you mean by a block? Did you ask the credit bureaus to report any use of the SSN? You can have them put a warning on any request for info.