You will need to enter it to claim tax credits. If you can't locate it, file an extension. You’ll need to search through your personal records to find your SSN or request a new Social Security card.
If you have a copy of a tax return where you claimed your son as a dependent, the SSN may be listed there. Check school and doctor records too. If you can't locate either of those, you can contact the Social Security Administration (SSA) to request a replacement card or verification of the SSN.