From @xmasbaby (above):
You can look at you own past tax returns to find your child's SSN or, as already suggested multiple times in this thread, you can check to see if his SSN is on medical or school records etc or you can request a new card from Social Security.
For prior year returns: You have to access your own account and/or print it for yourself using exactly the same account and user ID that you used when you prepared the return. See How do I access my prior-year returns?
Go to this Social Security website for the procedure - How do I replace a Social Security card for a child?"
@210SMOKEY210
