Call the IRS and order transcripts for tax years going back to 1995. These are free.
If he filed anything using your SSN, it will appear on your master tax record.
Don't say anything to the IRS when you make the call, just order the transcripts. They will be sent to you in about 30 days and you will be able to see what they have on record.
This post does not create an agreement to represent you before the IRS, nor does it invoke confidentiality regulations. Postings are based only on the information provided and you should consult a tax professional in your area before relying on information contained in this post.