How do I find out what my son’s SS# is when I can’t find the card?

[JohnnyB11]

If you filed a previous return with that info, their number will be on that tax return.  You can order a replacement SS card online (does not always work, but worth a try).
https://faq.ssa.gov/link/portal/34011/34019/Article/3873/How-do-I-replace-a-Social-Security-card-for...

[imanicelady34]

you will have to go into your local office with proof he or she is your child and they will give you a print out of your kids number