I believe you are correct.
The instructions for form 4868 say to mail the form, and if you owe, send money.
See https://www.irs.gov/pub/irs-pdf/f4868.pdf
For you - mail only the form 4868.
For you son - mail the form 4868 plus a check.
To avoid confusion, I would suggest mailing them in separate envelopes.
You do not need to send your 1040 forms or any other documents.
Get them postmarked by today.
